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NAME
re - Perl-like regular expressions for Erlang.
DESCRIPTION
This module contains regular expression matching functions for strings and binaries.
The regular expression syntax and semantics resemble that of Perl.
The matching algorithms of the library are based on the PCRE library, but not all of the
PCRE library is interfaced and some parts of the library go beyond what PCRE offers.
Currently PCRE version 8.40 (release date 2017-01-11) is used. The sections of the PCRE
documentation that are relevant to this module are included here.
Note:
The Erlang literal syntax for strings uses the "\" (backslash) character as an escape
code. You need to escape backslashes in literal strings, both in your code and in the
shell, with an extra backslash, that is, "\\".
DATA TYPES
mp() = {re_pattern, term(), term(), term(), term()}
Opaque data type containing a compiled regular expression. mp() is guaranteed to be
a tuple() having the atom re_pattern as its first element, to allow for matching in
guards. The arity of the tuple or the content of the other fields can change in
future Erlang/OTP releases.
nl_spec() = cr | crlf | lf | anycrlf | any
compile_option() =
unicode |
anchored |
caseless |
dollar_endonly |
dotall |
extended |
firstline |
multiline |
no_auto_capture |
dupnames |
ungreedy |
{newline, nl_spec()} |
bsr_anycrlf |
bsr_unicode |
no_start_optimize |
ucp |
never_utf
EXPORTS
version() -> binary()
The return of this function is a string with the PCRE version of the system that
was used in the Erlang/OTP compilation.
compile(Regexp) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
The same as compile(Regexp,[])
compile(Regexp, Options) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata() | unicode:charlist()
Options = [Option]
Option = compile_option()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
Compiles a regular expression, with the syntax described below, into an internal
format to be used later as a parameter to run/2 and run/3.
Compiling the regular expression before matching is useful if the same expression
is to be used in matching against multiple subjects during the lifetime of the
program. Compiling once and executing many times is far more efficient than
compiling each time one wants to match.
When option unicode is specified, the regular expression is to be specified as a
valid Unicode charlist(), otherwise as any valid iodata().
Options:
unicode:
The regular expression is specified as a Unicode charlist() and the resulting
regular expression code is to be run against a valid Unicode charlist()
subject. Also consider option ucp when using Unicode characters.
anchored:
The pattern is forced to be "anchored", that is, it is constrained to match
only at the first matching point in the string that is searched (the "subject
string"). This effect can also be achieved by appropriate constructs in the
pattern itself.
caseless:
Letters in the pattern match both uppercase and lowercase letters. It is
equivalent to Perl option /i and can be changed within a pattern by a (?i)
option setting. Uppercase and lowercase letters are defined as in the ISO
8859-1 character set.
dollar_endonly:
A dollar metacharacter in the pattern matches only at the end of the subject
string. Without this option, a dollar also matches immediately before a newline
at the end of the string (but not before any other newlines). This option is
ignored if option multiline is specified. There is no equivalent option in
Perl, and it cannot be set within a pattern.
dotall:
A dot in the pattern matches all characters, including those indicating
newline. Without it, a dot does not match when the current position is at a
newline. This option is equivalent to Perl option /s and it can be changed
within a pattern by a (?s) option setting. A negative class, such as [^a],
always matches newline characters, independent of the setting of this option.
extended:
If this option is set, most white space characters in the pattern are totally
ignored except when escaped or inside a character class. However, white space
is not allowed within sequences such as (?> that introduce various
parenthesized subpatterns, nor within a numerical quantifier such as {1,3}.
However, ignorable white space is permitted between an item and a following
quantifier and between a quantifier and a following + that indicates
possessiveness.
White space did not used to include the VT character (code 11), because Perl
did not treat this character as white space. However, Perl changed at release
5.18, so PCRE followed at release 8.34, and VT is now treated as white space.
This also causes characters between an unescaped # outside a character class
and the next newline, inclusive, to be ignored. This is equivalent to Perl's /x
option, and it can be changed within a pattern by a (?x) option setting.
With this option, comments inside complicated patterns can be included.
However, notice that this applies only to data characters. Whitespace
characters can never appear within special character sequences in a pattern,
for example within sequence (?( that introduces a conditional subpattern.
firstline:
An unanchored pattern is required to match before or at the first newline in
the subject string, although the matched text can continue over the newline.
multiline:
By default, PCRE treats the subject string as consisting of a single line of
characters (even if it contains newlines). The "start of line" metacharacter
(^) matches only at the start of the string, while the "end of line"
metacharacter ($) matches only at the end of the string, or before a
terminating newline (unless option dollar_endonly is specified). This is the
same as in Perl.
When this option is specified, the "start of line" and "end of line" constructs
match immediately following or immediately before internal newlines in the
subject string, respectively, as well as at the very start and end. This is
equivalent to Perl option /m and can be changed within a pattern by a (?m)
option setting. If there are no newlines in a subject string, or no occurrences
of ^ or $ in a pattern, setting multiline has no effect.
no_auto_capture:
Disables the use of numbered capturing parentheses in the pattern. Any opening
parenthesis that is not followed by ? behaves as if it is followed by ?:. Named
parentheses can still be used for capturing (and they acquire numbers in the
usual way). There is no equivalent option in Perl.
dupnames:
Names used to identify capturing subpatterns need not be unique. This can be
helpful for certain types of pattern when it is known that only one instance of
the named subpattern can ever be matched. More details of named subpatterns are
provided below.
ungreedy:
Inverts the "greediness" of the quantifiers so that they are not greedy by
default, but become greedy if followed by "?". It is not compatible with Perl.
It can also be set by a (?U) option setting within the pattern.
{newline, NLSpec}:
Overrides the default definition of a newline in the subject string, which is
LF (ASCII 10) in Erlang.
cr:
Newline is indicated by a single character cr (ASCII 13).
lf:
Newline is indicated by a single character LF (ASCII 10), the default.
crlf:
Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII
10) sequence.
anycrlf:
Any of the three preceding sequences is to be recognized.
any:
Any of the newline sequences above, and the Unicode sequences VT (vertical
tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
separator, U+2028), and PS (paragraph separator, U+2029).
bsr_anycrlf:
Specifies specifically that \R is to match only the CR, LF, or CRLF sequences,
not the Unicode-specific newline characters.
bsr_unicode:
Specifies specifically that \R is to match all the Unicode newline characters
(including CRLF, and so on, the default).
no_start_optimize:
Disables optimization that can malfunction if "Special start-of-pattern items"
are present in the regular expression. A typical example would be when matching
"DEFABC" against "(*COMMIT)ABC", where the start optimization of PCRE would
skip the subject up to "A" and never realize that the (*COMMIT) instruction is
to have made the matching fail. This option is only relevant if you use "start-
of-pattern items", as discussed in section PCRE Regular Expression Details.
ucp:
Specifies that Unicode character properties are to be used when resolving \B,
\b, \D, \d, \S, \s, \W and \w. Without this flag, only ISO Latin-1 properties
are used. Using Unicode properties hurts performance, but is semantically
correct when working with Unicode characters beyond the ISO Latin-1 range.
never_utf:
Specifies that the (*UTF) and/or (*UTF8) "start-of-pattern items" are
forbidden. This flag cannot be combined with option unicode. Useful if ISO
Latin-1 patterns from an external source are to be compiled.
inspect(MP, Item) -> {namelist, [binary()]}
Types:
MP = mp()
Item = namelist
Takes a compiled regular expression and an item, and returns the relevant data from
the regular expression. The only supported item is namelist, which returns the
tuple {namelist, [binary()]}, containing the names of all (unique) named
subpatterns in the regular expression. For example:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
2> re:inspect(MP,namelist).
{namelist,[<<"A">>,<<"B">>,<<"C">>]}
3> {ok,MPD} = re:compile("(?<C>A)|(?<B>B)|(?<C>C)",[dupnames]).
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,8,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
4> re:inspect(MPD,namelist).
{namelist,[<<"B">>,<<"C">>]}
Notice in the second example that the duplicate name only occurs once in the
returned list, and that the list is in alphabetical order regardless of where the
names are positioned in the regular expression. The order of the names is the same
as the order of captured subexpressions if {capture, all_names} is specified as an
option to run/3. You can therefore create a name-to-value mapping from the result
of run/3 like this:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
2> {namelist, N} = re:inspect(MP,namelist).
{namelist,[<<"A">>,<<"B">>,<<"C">>]}
3> {match,L} = re:run("AA",MP,[{capture,all_names,binary}]).
{match,[<<"A">>,<<>>,<<>>]}
4> NameMap = lists:zip(N,L).
[{<<"A">>,<<"A">>},{<<"B">>,<<>>},{<<"C">>,<<>>}]
replace(Subject, RE, Replacement) -> iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Replacement = iodata() | unicode:charlist()
Same as replace(Subject, RE, Replacement, []).
replace(Subject, RE, Replacement, Options) ->
iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Replacement = iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
global |
notbol |
noteol |
notempty |
notempty_atstart |
{offset, integer() >= 0} |
{newline, NLSpec} |
bsr_anycrlf |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
bsr_unicode |
{return, ReturnType} |
CompileOpt
ReturnType = iodata | list | binary
CompileOpt = compile_option()
NLSpec = cr | crlf | lf | anycrlf | any
Replaces the matched part of the Subject string with the contents of Replacement.
The permissible options are the same as for run/3, except that option capture is
not allowed. Instead a {return, ReturnType} is present. The default return type is
iodata, constructed in a way to minimize copying. The iodata result can be used
directly in many I/O operations. If a flat list() is desired, specify {return,
list}. If a binary is desired, specify {return, binary}.
As in function run/3, an mp() compiled with option unicode requires Subject to be a
Unicode charlist(). If compilation is done implicitly and the unicode compilation
option is specified to this function, both the regular expression and Subject are
to specified as valid Unicode charlist()s.
The replacement string can contain the special character &, which inserts the whole
matching expression in the result, and the special sequence \N (where N is an
integer > 0), \gN, or \g{N}, resulting in the subexpression number N, is inserted
in the result. If no subexpression with that number is generated by the regular
expression, nothing is inserted.
To insert an & or a \ in the result, precede it with a \. Notice that Erlang
already gives a special meaning to \ in literal strings, so a single \ must be
written as "\\" and therefore a double \ as "\\\\".
Example:
re:replace("abcd","c","[&]",[{return,list}]).
gives
"ab[c]d"
while
re:replace("abcd","c","[\\&]",[{return,list}]).
gives
"ab[&]d"
As with run/3, compilation errors raise the badarg exception. compile/2 can be used
to get more information about the error.
run(Subject, RE) -> {match, Captured} | nomatch
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Captured = [CaptureData]
CaptureData = {integer(), integer()}
Same as run(Subject,RE,[]).
run(Subject, RE, Options) ->
{match, Captured} | match | nomatch | {error, ErrType}
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
global |
notbol |
noteol |
notempty |
notempty_atstart |
report_errors |
{offset, integer() >= 0} |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
{newline, NLSpec :: nl_spec()} |
bsr_anycrlf |
bsr_unicode |
{capture, ValueSpec} |
{capture, ValueSpec, Type} |
CompileOpt
Type = index | list | binary
ValueSpec =
all | all_but_first | all_names | first | none | ValueList
ValueList = [ValueID]
ValueID = integer() | string() | atom()
CompileOpt = compile_option()
See compile/2.
Captured = [CaptureData] | [[CaptureData]]
CaptureData =
{integer(), integer()} | ListConversionData | binary()
ListConversionData =
string() |
{error, string(), binary()} |
{incomplete, string(), binary()}
ErrType =
match_limit | match_limit_recursion | {compile, CompileErr}
CompileErr =
{ErrString :: string(), Position :: integer() >= 0}
Executes a regular expression matching, and returns match/{match, Captured} or
nomatch. The regular expression can be specified either as iodata() in which case
it is automatically compiled (as by compile/2) and executed, or as a precompiled
mp() in which case it is executed against the subject directly.
When compilation is involved, exception badarg is thrown if a compilation error
occurs. Call compile/2 to get information about the location of the error in the
regular expression.
If the regular expression is previously compiled, the option list can only contain
the following options:
* anchored
* {capture, ValueSpec}/{capture, ValueSpec, Type}
* global
* {match_limit, integer() >= 0}
* {match_limit_recursion, integer() >= 0}
* {newline, NLSpec}
* notbol
* notempty
* notempty_atstart
* noteol
* {offset, integer() >= 0}
* report_errors
Otherwise all options valid for function compile/2 are also allowed. Options
allowed both for compilation and execution of a match, namely anchored and
{newline, NLSpec}, affect both the compilation and execution if present together
with a non-precompiled regular expression.
If the regular expression was previously compiled with option unicode, Subject is
to be provided as a valid Unicode charlist(), otherwise any iodata() will do. If
compilation is involved and option unicode is specified, both Subject and the
regular expression are to be specified as valid Unicode charlists().
{capture, ValueSpec}/{capture, ValueSpec, Type} defines what to return from the
function upon successful matching. The capture tuple can contain both a value
specification, telling which of the captured substrings are to be returned, and a
type specification, telling how captured substrings are to be returned (as index
tuples, lists, or binaries). The options are described in detail below.
If the capture options describe that no substring capturing is to be done
({capture, none}), the function returns the single atom match upon successful
matching, otherwise the tuple {match, ValueList}. Disabling capturing can be done
either by specifying none or an empty list as ValueSpec.
Option report_errors adds the possibility that an error tuple is returned. The
tuple either indicates a matching error (match_limit or match_limit_recursion), or
a compilation error, where the error tuple has the format {error, {compile,
CompileErr}}. Notice that if option report_errors is not specified, the function
never returns error tuples, but reports compilation errors as a badarg exception
and failed matches because of exceeded match limits simply as nomatch.
The following options are relevant for execution:
anchored:
Limits run/3 to matching at the first matching position. If a pattern was
compiled with anchored, or turned out to be anchored by virtue of its contents,
it cannot be made unanchored at matching time, hence there is no unanchored
option.
global:
Implements global (repetitive) search (flag g in Perl). Each match is returned
as a separate list() containing the specific match and any matching
subexpressions (or as specified by option capture. The Captured part of the
return value is hence a list() of list()s when this option is specified.
The interaction of option global with a regular expression that matches an
empty string surprises some users. When option global is specified, run/3
handles empty matches in the same way as Perl: a zero-length match at any point
is also retried with options [anchored, notempty_atstart]. If that search gives
a result of length > 0, the result is included. Example:
re:run("cat","(|at)",[global]).
The following matchings are performed:
At offset 0:
The regular expression (|at) first match at the initial position of string
cat, giving the result set [{0,0},{0,0}] (the second {0,0} is because of the
subexpression marked by the parentheses). As the length of the match is 0, we
do not advance to the next position yet.
At offset 0 with [anchored, notempty_atstart]:
The search is retried with options [anchored, notempty_atstart] at the same
position, which does not give any interesting result of longer length, so the
search position is advanced to the next character (a).
At offset 1:
The search results in [{1,0},{1,0}], so this search is also repeated with the
extra options.
At offset 1 with [anchored, notempty_atstart]:
Alternative ab is found and the result is [{1,2},{1,2}]. The result is added
to the list of results and the position in the search string is advanced two
steps.
At offset 3:
The search once again matches the empty string, giving [{3,0},{3,0}].
At offset 1 with [anchored, notempty_atstart]:
This gives no result of length > 0 and we are at the last position, so the
global search is complete.
The result of the call is:
{match,[[{0,0},{0,0}],[{1,0},{1,0}],[{1,2},{1,2}],[{3,0},{3,0}]]}
notempty:
An empty string is not considered to be a valid match if this option is
specified. If alternatives in the pattern exist, they are tried. If all the
alternatives match the empty string, the entire match fails.
Example:
If the following pattern is applied to a string not beginning with "a" or "b",
it would normally match the empty string at the start of the subject:
a?b?
With option notempty, this match is invalid, so run/3 searches further into the
string for occurrences of "a" or "b".
notempty_atstart:
Like notempty, except that an empty string match that is not at the start of
the subject is permitted. If the pattern is anchored, such a match can occur
only if the pattern contains \K.
Perl has no direct equivalent of notempty or notempty_atstart, but it does make
a special case of a pattern match of the empty string within its split()
function, and when using modifier /g. The Perl behavior can be emulated after
matching a null string by first trying the match again at the same offset with
notempty_atstart and anchored, and then, if that fails, by advancing the
starting offset (see below) and trying an ordinary match again.
notbol:
Specifies that the first character of the subject string is not the beginning
of a line, so the circumflex metacharacter is not to match before it. Setting
this without multiline (at compile time) causes circumflex never to match. This
option only affects the behavior of the circumflex metacharacter. It does not
affect \\A.
noteol:
Specifies that the end of the subject string is not the end of a line, so the
dollar metacharacter is not to match it nor (except in multiline mode) a
newline immediately before it. Setting this without multiline (at compile time)
causes dollar never to match. This option affects only the behavior of the
dollar metacharacter. It does not affect \\Z or \\z.
report_errors:
Gives better control of the error handling in run/3. When specified,
compilation errors (if the regular expression is not already compiled) and
runtime errors are explicitly returned as an error tuple.
The following are the possible runtime errors:
match_limit:
The PCRE library sets a limit on how many times the internal match function
can be called. Defaults to 10,000,000 in the library compiled for Erlang. If
{error, match_limit} is returned, the execution of the regular expression has
reached this limit. This is normally to be regarded as a nomatch, which is
the default return value when this occurs, but by specifying report_errors,
you are informed when the match fails because of too many internal calls.
match_limit_recursion:
This error is very similar to match_limit, but occurs when the internal match
function of PCRE is "recursively" called more times than the
match_limit_recursion limit, which defaults to 10,000,000 as well. Notice
that as long as the match_limit and match_limit_default values are kept at
the default values, the match_limit_recursion error cannot occur, as the
match_limit error occurs before that (each recursive call is also a call, but
not conversely). Both limits can however be changed, either by setting limits
directly in the regular expression string (see section PCRE Regular
Eexpression Details) or by specifying options to run/3.
It is important to understand that what is referred to as "recursion" when
limiting matches is not recursion on the C stack of the Erlang machine or on
the Erlang process stack. The PCRE version compiled into the Erlang VM uses
machine "heap" memory to store values that must be kept over recursion in
regular expression matches.
{match_limit, integer() >= 0}:
Limits the execution time of a match in an implementation-specific way. It is
described as follows by the PCRE documentation:
The match_limit field provides a means of preventing PCRE from using
up a vast amount of resources when running patterns that are not going
to match, but which have a very large number of possibilities in their
search trees. The classic example is a pattern that uses nested
unlimited repeats.
Internally, pcre_exec() uses a function called match(), which it calls
repeatedly (sometimes recursively). The limit set by match_limit is
imposed on the number of times this function is called during a match,
which has the effect of limiting the amount of backtracking that can
take place. For patterns that are not anchored, the count restarts
from zero for each position in the subject string.
This means that runaway regular expression matches can fail faster if the limit
is lowered using this option. The default value 10,000,000 is compiled into the
Erlang VM.
Note:
This option does in no way affect the execution of the Erlang VM in terms of
"long running BIFs". run/3 always gives control back to the scheduler of Erlang
processes at intervals that ensures the real-time properties of the Erlang
system.
{match_limit_recursion, integer() >= 0}:
Limits the execution time and memory consumption of a match in an
implementation-specific way, very similar to match_limit. It is described as
follows by the PCRE documentation:
The match_limit_recursion field is similar to match_limit, but instead
of limiting the total number of times that match() is called, it
limits the depth of recursion. The recursion depth is a smaller number
than the total number of calls, because not all calls to match() are
recursive. This limit is of use only if it is set smaller than
match_limit.
Limiting the recursion depth limits the amount of machine stack that
can be used, or, when PCRE has been compiled to use memory on the heap
instead of the stack, the amount of heap memory that can be used.
The Erlang VM uses a PCRE library where heap memory is used when regular
expression match recursion occurs. This therefore limits the use of machine
heap, not C stack.
Specifying a lower value can result in matches with deep recursion failing,
when they should have matched:
1> re:run("aaaaaaaaaaaaaz","(a+)*z").
{match,[{0,14},{0,13}]}
2> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5}]).
nomatch
3> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5},report_errors]).
{error,match_limit_recursion}
This option and option match_limit are only to be used in rare cases.
Understanding of the PCRE library internals is recommended before tampering
with these limits.
{offset, integer() >= 0}:
Start matching at the offset (position) specified in the subject string. The
offset is zero-based, so that the default is {offset,0} (all of the subject
string).
{newline, NLSpec}:
Overrides the default definition of a newline in the subject string, which is
LF (ASCII 10) in Erlang.
cr:
Newline is indicated by a single character CR (ASCII 13).
lf:
Newline is indicated by a single character LF (ASCII 10), the default.
crlf:
Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII
10) sequence.
anycrlf:
Any of the three preceding sequences is be recognized.
any:
Any of the newline sequences above, and the Unicode sequences VT (vertical
tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
separator, U+2028), and PS (paragraph separator, U+2029).
bsr_anycrlf:
Specifies specifically that \R is to match only the CR LF, or CRLF sequences,
not the Unicode-specific newline characters. (Overrides the compilation
option.)
bsr_unicode:
Specifies specifically that \R is to match all the Unicode newline characters
(including CRLF, and so on, the default). (Overrides the compilation option.)
{capture, ValueSpec}/{capture, ValueSpec, Type}:
Specifies which captured substrings are returned and in what format. By
default, run/3 captures all of the matching part of the substring and all
capturing subpatterns (all of the pattern is automatically captured). The
default return type is (zero-based) indexes of the captured parts of the
string, specified as {Offset,Length} pairs (the index Type of capturing).
As an example of the default behavior, the following call returns, as first and
only captured string, the matching part of the subject ("abcd" in the middle)
as an index pair {3,4}, where character positions are zero-based, just as in
offsets:
re:run("ABCabcdABC","abcd",[]).
The return value of this call is:
{match,[{3,4}]}
Another (and quite common) case is where the regular expression matches all of
the subject:
re:run("ABCabcdABC",".*abcd.*",[]).
Here the return value correspondingly points out all of the string, beginning
at index 0, and it is 10 characters long:
{match,[{0,10}]}
If the regular expression contains capturing subpatterns, like in:
re:run("ABCabcdABC",".*(abcd).*",[]).
all of the matched subject is captured, as well as the captured substrings:
{match,[{0,10},{3,4}]}
The complete matching pattern always gives the first return value in the list
and the remaining subpatterns are added in the order they occurred in the
regular expression.
The capture tuple is built up as follows:
ValueSpec:
Specifies which captured (sub)patterns are to be returned. ValueSpec can
either be an atom describing a predefined set of return values, or a list
containing the indexes or the names of specific subpatterns to return.
The following are the predefined sets of subpatterns:
all:
All captured subpatterns including the complete matching string. This is
the default.
all_names:
All named subpatterns in the regular expression, as if a list() of all the
names in alphabetical order was specified. The list of all names can also
be retrieved with inspect/2.
first:
Only the first captured subpattern, which is always the complete matching
part of the subject. All explicitly captured subpatterns are discarded.
all_but_first:
All but the first matching subpattern, that is, all explicitly captured
subpatterns, but not the complete matching part of the subject string. This
is useful if the regular expression as a whole matches a large part of the
subject, but the part you are interested in is in an explicitly captured
subpattern. If the return type is list or binary, not returning subpatterns
you are not interested in is a good way to optimize.
none:
Returns no matching subpatterns, gives the single atom match as the return
value of the function when matching successfully instead of the {match,
list()} return. Specifying an empty list gives the same behavior.
The value list is a list of indexes for the subpatterns to return, where
index 0 is for all of the pattern, and 1 is for the first explicit capturing
subpattern in the regular expression, and so on. When using named captured
subpatterns (see below) in the regular expression, one can use atom()s or
string()s to specify the subpatterns to be returned. For example, consider
the regular expression:
".*(abcd).*"
matched against string "ABCabcdABC", capturing only the "abcd" part (the
first explicit subpattern):
re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).
The call gives the following result, as the first explicitly captured
subpattern is "(abcd)", matching "abcd" in the subject, at (zero-based)
position 3, of length 4:
{match,[{3,4}]}
Consider the same regular expression, but with the subpattern explicitly
named 'FOO':
".*(?<FOO>abcd).*"
With this expression, we could still give the index of the subpattern with
the following call:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).
giving the same result as before. But, as the subpattern is named, we can
also specify its name in the value list:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).
This would give the same result as the earlier examples, namely:
{match,[{3,4}]}
The values list can specify indexes or names not present in the regular
expression, in which case the return values vary depending on the type. If
the type is index, the tuple {-1,0} is returned for values with no
corresponding subpattern in the regular expression, but for the other types
(binary and list), the values are the empty binary or list, respectively.
Type:
Optionally specifies how captured substrings are to be returned. If omitted,
the default of index is used.
Type can be one of the following:
index:
Returns captured substrings as pairs of byte indexes into the subject
string and length of the matching string in the subject (as if the subject
string was flattened with erlang:iolist_to_binary/1 or
unicode:characters_to_binary/2 before matching). Notice that option unicode
results in byte-oriented indexes in a (possibly virtual) UTF-8 encoded
binary. A byte index tuple {0,2} can therefore represent one or two
characters when unicode is in effect. This can seem counter-intuitive, but
has been deemed the most effective and useful way to do it. To return lists
instead can result in simpler code if that is desired. This return type is
the default.
list:
Returns matching substrings as lists of characters (Erlang string()s). It
option unicode is used in combination with the \C sequence in the regular
expression, a captured subpattern can contain bytes that are not valid
UTF-8 (\C matches bytes regardless of character encoding). In that case the
list capturing can result in the same types of tuples that
unicode:characters_to_list/2 can return, namely three-tuples with tag
incomplete or error, the successfully converted characters and the invalid
UTF-8 tail of the conversion as a binary. The best strategy is to avoid
using the \C sequence when capturing lists.
binary:
Returns matching substrings as binaries. If option unicode is used, these
binaries are in UTF-8. If the \C sequence is used together with unicode,
the binaries can be invalid UTF-8.
In general, subpatterns that were not assigned a value in the match are
returned as the tuple {-1,0} when type is index. Unassigned subpatterns are
returned as the empty binary or list, respectively, for other return types.
Consider the following regular expression:
".*((?<FOO>abdd)|a(..d)).*"
There are three explicitly capturing subpatterns, where the opening parenthesis
position determines the order in the result, hence ((?<FOO>abdd)|a(..d)) is
subpattern index 1, (?<FOO>abdd) is subpattern index 2, and (..d) is subpattern
index 3. When matched against the following string:
"ABCabcdABC"
the subpattern at index 2 does not match, as "abdd" is not present in the
string, but the complete pattern matches (because of the alternative a(..d)).
The subpattern at index 2 is therefore unassigned and the default return value
is:
{match,[{0,10},{3,4},{-1,0},{4,3}]}
Setting the capture Type to binary gives:
{match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}
Here the empty binary (<<>>) represents the unassigned subpattern. In the
binary case, some information about the matching is therefore lost, as <<>> can
also be an empty string captured.
If differentiation between empty matches and non-existing subpatterns is
necessary, use the type index and do the conversion to the final type in Erlang
code.
When option global is speciified, the capture specification affects each match
separately, so that:
re:run("cacb","c(a|b)",[global,{capture,[1],list}]).
gives
{match,[["a"],["b"]]}
For a descriptions of options only affecting the compilation step, see compile/2.
split(Subject, RE) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
SplitList = [iodata() | unicode:charlist()]
Same as split(Subject, RE, []).
split(Subject, RE, Options) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
notbol |
noteol |
notempty |
notempty_atstart |
{offset, integer() >= 0} |
{newline, nl_spec()} |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
bsr_anycrlf |
bsr_unicode |
{return, ReturnType} |
{parts, NumParts} |
group |
trim |
CompileOpt
NumParts = integer() >= 0 | infinity
ReturnType = iodata | list | binary
CompileOpt = compile_option()
See compile/2.
SplitList = [RetData] | [GroupedRetData]
GroupedRetData = [RetData]
RetData = iodata() | unicode:charlist() | binary() | list()
Splits the input into parts by finding tokens according to the regular expression
supplied. The splitting is basically done by running a global regular expression
match and dividing the initial string wherever a match occurs. The matching part of
the string is removed from the output.
As in run/3, an mp() compiled with option unicode requires Subject to be a Unicode
charlist(). If compilation is done implicitly and the unicode compilation option is
specified to this function, both the regular expression and Subject are to be
specified as valid Unicode charlist()s.
The result is given as a list of "strings", the preferred data type specified in
option return (default iodata).
If subexpressions are specified in the regular expression, the matching
subexpressions are returned in the resulting list as well. For example:
re:split("Erlang","[ln]",[{return,list}]).
gives
["Er","a","g"]
while
re:split("Erlang","([ln])",[{return,list}]).
gives
["Er","l","a","n","g"]
The text matching the subexpression (marked by the parentheses in the regular
expression) is inserted in the result list where it was found. This means that
concatenating the result of a split where the whole regular expression is a single
subexpression (as in the last example) always results in the original string.
As there is no matching subexpression for the last part in the example (the "g"),
nothing is inserted after that. To make the group of strings and the parts matching
the subexpressions more obvious, one can use option group, which groups together
the part of the subject string with the parts matching the subexpressions when the
string was split:
re:split("Erlang","([ln])",[{return,list},group]).
gives
[["Er","l"],["a","n"],["g"]]
Here the regular expression first matched the "l", causing "Er" to be the first
part in the result. When the regular expression matched, the (only) subexpression
was bound to the "l", so the "l" is inserted in the group together with "Er". The
next match is of the "n", making "a" the next part to be returned. As the
subexpression is bound to substring "n" in this case, the "n" is inserted into this
group. The last group consists of the remaining string, as no more matches are
found.
By default, all parts of the string, including the empty strings, are returned from
the function, for example:
re:split("Erlang","[lg]",[{return,list}]).
gives
["Er","an",[]]
as the matching of the "g" in the end of the string leaves an empty rest, which is
also returned. This behavior differs from the default behavior of the split
function in Perl, where empty strings at the end are by default removed. To get the
"trimming" default behavior of Perl, specify trim as an option:
re:split("Erlang","[lg]",[{return,list},trim]).
gives
["Er","an"]
The "trim" option says; "give me as many parts as possible except the empty ones",
which sometimes can be useful. You can also specify how many parts you want, by
specifying {parts,N}:
re:split("Erlang","[lg]",[{return,list},{parts,2}]).
gives
["Er","ang"]
Notice that the last part is "ang", not "an", as splitting was specified into two
parts, and the splitting stops when enough parts are given, which is why the result
differs from that of trim.
More than three parts are not possible with this indata, so
re:split("Erlang","[lg]",[{return,list},{parts,4}]).
gives the same result as the default, which is to be viewed as "an infinite number
of parts".
Specifying 0 as the number of parts gives the same effect as option trim. If
subexpressions are captured, empty subexpressions matched at the end are also
stripped from the result if trim or {parts,0} is specified.
The trim behavior corresponds exactly to the Perl default. {parts,N}, where N is a
positive integer, corresponds exactly to the Perl behavior with a positive
numerical third parameter. The default behavior of split/3 corresponds to the Perl
behavior when a negative integer is specified as the third parameter for the Perl
routine.
Summary of options not previously described for function run/3:
{return,ReturnType}:
Specifies how the parts of the original string are presented in the result
list. Valid types:
iodata:
The variant of iodata() that gives the least copying of data with the current
implementation (often a binary, but do not depend on it).
binary:
All parts returned as binaries.
list:
All parts returned as lists of characters ("strings").
group:
Groups together the part of the string with the parts of the string matching
the subexpressions of the regular expression.
The return value from the function is in this case a list() of list()s. Each
sublist begins with the string picked out of the subject string, followed by
the parts matching each of the subexpressions in order of occurrence in the
regular expression.
{parts,N}:
Specifies the number of parts the subject string is to be split into.
The number of parts is to be a positive integer for a specific maximum number
of parts, and infinity for the maximum number of parts possible (the default).
Specifying {parts,0} gives as many parts as possible disregarding empty parts
at the end, the same as specifying trim.
trim:
Specifies that empty parts at the end of the result list are to be disregarded.
The same as specifying {parts,0}. This corresponds to the default behavior of
the split built-in function in Perl.
PERL-LIKE REGULAR EXPRESSION SYNTAX
The following sections contain reference material for the regular expressions used by this
module. The information is based on the PCRE documentation, with changes where this module
behaves differently to the PCRE library.
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE are described in
detail in the following sections. Perl's regular expressions are described in its own
documentation, and regular expressions in general are covered in many books, some with
copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly,
covers regular expressions in great detail. This description of the PCRE regular
expressions is intended as reference material.
The reference material is divided into the following sections:
* Special Start-of-Pattern Items
* Characters and Metacharacters
* Backslash
* Circumflex and Dollar
* Full Stop (Period, Dot) and \N
* Matching a Single Data Unit
* Square Brackets and Character Classes
* Posix Character Classes
* Vertical Bar
* Internal Option Setting
* Subpatterns
* Duplicate Subpattern Numbers
* Named Subpatterns
* Repetition
* Atomic Grouping and Possessive Quantifiers
* Back References
* Assertions
* Conditional Subpatterns
* Comments
* Recursive Patterns
* Subpatterns as Subroutines
* Oniguruma Subroutine Syntax
* Backtracking Control
SPECIAL START-OF-PATTERN ITEMS
Some options that can be passed to compile/2 can also be set by special items at the start
of a pattern. These are not Perl-compatible, but are provided to make these options
accessible to pattern writers who are not able to change the program that processes the
pattern. Any number of these items can appear, but they must all be together right at the
start of the pattern string, and the letters must be in upper case.
UTF Support
Unicode support is basically UTF-8 based. To use Unicode characters, you either call
compile/2 or run/3 with option unicode, or the pattern must start with one of these
special sequences:
(*UTF8)
(*UTF)
Both options give the same effect, the input string is interpreted as UTF-8. Notice that
with these instructions, the automatic conversion of lists to UTF-8 is not performed by
the re functions. Therefore, using these sequences is not recommended. Add option unicode
when running compile/2 instead.
Some applications that allow their users to supply patterns can wish to restrict them to
non-UTF data for security reasons. If option never_utf is set at compile time, (*UTF), and
so on, are not allowed, and their appearance causes an error.
Unicode Property Support
The following is another special sequence that can appear at the start of a pattern:
(*UCP)
This has the same effect as setting option ucp: it causes sequences such as \d and \w to
use Unicode properties to determine character types, instead of recognizing only
characters with codes < 256 through a lookup table.
Disabling Startup Optimizations
If a pattern starts with (*NO_START_OPT), it has the same effect as setting option
no_start_optimize at compile time.
Newline Conventions
PCRE supports five conventions for indicating line breaks in strings: a single CR
(carriage return) character, a single LF (line feed) character, the two-character sequence
CRLF, any of the three preceding, and any Unicode newline sequence.
A newline convention can also be specified by starting a pattern string with one of the
following five sequences:
(*CR):
Carriage return
(*LF):
Line feed
(*CRLF):
>Carriage return followed by line feed
(*ANYCRLF):
Any of the three above
(*ANY):
All Unicode newline sequences
These override the default and the options specified to compile/2. For example, the
following pattern changes the convention to CR:
(*CR)a.b
This pattern matches a\nb, as LF is no longer a newline. If more than one of them is
present, the last one is used.
The newline convention affects where the circumflex and dollar assertions are true. It
also affects the interpretation of the dot metacharacter when dotall is not set, and the
behavior of \N. However, it does not affect what the \R escape sequence matches. By
default, this is any Unicode newline sequence, for Perl compatibility. However, this can
be changed; see the description of \R in section Newline Sequences. A change of the \R
setting can be combined with a change of the newline convention.
Setting Match and Recursion Limits
The caller of run/3 can set a limit on the number of times the internal match() function
is called and on the maximum depth of recursive calls. These facilities are provided to
catch runaway matches that are provoked by patterns with huge matching trees (a typical
example is a pattern with nested unlimited repeats) and to avoid running out of system
stack by too much recursion. When one of these limits is reached, pcre_exec() gives an
error return. The limits can also be set by items at the start of the pattern of the
following forms:
(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)
Here d is any number of decimal digits. However, the value of the setting must be less
than the value set by the caller of run/3 for it to have any effect. That is, the pattern
writer can lower the limit set by the programmer, but not raise it. If there is more than
one setting of one of these limits, the lower value is used.
The default value for both the limits is 10,000,000 in the Erlang VM. Notice that the
recursion limit does not affect the stack depth of the VM, as PCRE for Erlang is compiled
in such a way that the match function never does recursion on the C stack.
Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by
the caller, not increase them.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject string from left to
right. Most characters stand for themselves in a pattern and match the corresponding
characters in the subject. As a trivial example, the following pattern matches a portion
of a subject string that is identical to itself:
The quick brown fox
When caseless matching is specified (option caseless), letters are matched independently
of case.
The power of regular expressions comes from the ability to include alternatives and
repetitions in the pattern. These are encoded in the pattern by the use of metacharacters,
which do not stand for themselves but instead are interpreted in some special way.
Two sets of metacharacters exist: those that are recognized anywhere in the pattern except
within square brackets, and those that are recognized within square brackets. Outside
square brackets, the metacharacters are as follows:
\:
General escape character with many uses
^:
Assert start of string (or line, in multiline mode)
$:
Assert end of string (or line, in multiline mode)
.:
Match any character except newline (by default)
[:
Start character class definition
|:
Start of alternative branch
(:
Start subpattern
):
End subpattern
?:
Extends the meaning of (, also 0 or 1 quantifier, also quantifier minimizer
*:
0 or more quantifiers
+:
1 or more quantifier, also "possessive quantifier"
{:
Start min/max quantifier
Part of a pattern within square brackets is called a "character class". The following are
the only metacharacters in a character class:
\:
General escape character
^:
Negate the class, but only if the first character
-:
Indicates character range
[:
Posix character class (only if followed by Posix syntax)
]:
Terminates the character class
The following sections describe the use of each metacharacter.
BACKSLASH
The backslash character has many uses. First, if it is followed by a character that is not
a number or a letter, it takes away any special meaning that a character can have. This
use of backslash as an escape character applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the pattern. This
escaping action applies if the following character would otherwise be interpreted as a
metacharacter, so it is always safe to precede a non-alphanumeric with backslash to
specify that it stands for itself. In particular, if you want to match a backslash, write
\\.
In unicode mode, only ASCII numbers and letters have any special meaning after a
backslash. All other characters (in particular, those whose code points are > 127) are
treated as literals.
If a pattern is compiled with option extended, whitespace in the pattern (other than in a
character class) and characters between a # outside a character class and the next newline
are ignored. An escaping backslash can be used to include a whitespace or # character as
part of the pattern.
To remove the special meaning from a sequence of characters, put them between \Q and \E.
This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences
in PCRE, while $ and @ cause variable interpolation in Perl. Notice the following
examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character classes. An isolated
\E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the
pattern, the literal interpretation continues to the end of the pattern (that is, \E is
assumed at the end). If the isolated \Q is inside a character class, this causes an error,
as the character class is not terminated.
Non-Printing Characters
A second use of backslash provides a way of encoding non-printing characters in patterns
in a visible manner. There is no restriction on the appearance of non-printing characters,
apart from the binary zero that terminates a pattern. When a pattern is prepared by text
editing, it is often easier to use one of the following escape sequences than the binary
character it represents:
\a:
Alarm, that is, the BEL character (hex 07)
\cx:
"Control-x", where x is any ASCII character
\e:
Escape (hex 1B)
\f:
Form feed (hex 0C)
\n:
Line feed (hex 0A)
\r:
Carriage return (hex 0D)
\t:
Tab (hex 09)
\0dd:
Character with octal code 0dd
\ddd:
Character with octal code ddd, or back reference
\o{ddd..}:
character with octal code ddd..
\xhh:
Character with hex code hh
\x{hhh..}:
Character with hex code hhh..
Note:
Note that \0dd is always an octal code, and that \8 and \9 are the literal characters "8"
and "9".
The precise effect of \cx on ASCII characters is as follows: if x is a lowercase letter,
it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA
to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and
\c; becomes hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c has a
value > 127, a compile-time error occurs. This locks out non-ASCII characters in all
modes.
The \c facility was designed for use with ASCII characters, but with the extension to
Unicode it is even less useful than it once was.
After \0 up to two further octal digits are read. If there are fewer than two digits, just
those that are present are used. Thus the sequence \0\x\015 specifies two binary zeros
followed by a CR character (code value 13). Make sure you supply two digits after the
initial zero if the pattern character that follows is itself an octal digit.
The escape \o must be followed by a sequence of octal digits, enclosed in braces. An error
occurs if this is not the case. This escape is a recent addition to Perl; it provides way
of specifying character code points as octal numbers greater than 0777, and it also allows
octal numbers and back references to be unambiguously specified.
For greater clarity and unambiguity, it is best to avoid following \ by a digit greater
than zero. Instead, use \o{} or \x{} to specify character numbers, and \g{} to specify
back references. The following paragraphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is complicated, and Perl has
changed in recent releases, causing PCRE also to change. Outside a character class, PCRE
reads the digit and any following digits as a decimal number. If the number is < 8, or if
there have been at least that many previous capturing left parentheses in the expression,
the entire sequence is taken as a back reference. A description of how this works is
provided later, following the discussion of parenthesized subpatterns.
Inside a character class, or if the decimal number following \ is > 7 and there have not
been that many capturing subpatterns, PCRE handles \8 and \9 as the literal characters "8"
and "9", and otherwise re-reads up to three octal digits following the backslash, and
using them to generate a data character. Any subsequent digits stand for themselves. For
example:
\040:
Another way of writing an ASCII space
\40:
The same, provided there are < 40 previous capturing subpatterns
\7:
Always a back reference
\11:
Can be a back reference, or another way of writing a tab
\011:
Always a tab
\0113:
A tab followed by character "3"
\113:
Can be a back reference, otherwise the character with octal code 113
\377:
Can be a back reference, otherwise value 255 (decimal)
\81:
Either a back reference, or the two characters "8" and "1"
Notice that octal values >= 100 that are specified using this syntax must not be
introduced by a leading zero, as no more than three octal digits are ever read.
By default, after \x that is not followed by {, from zero to two hexadecimal digits are
read (letters can be in upper or lower case). Any number of hexadecimal digits may appear
between \x{ and }. If a character other than a hexadecimal digit appears between \x{ and
}, or if there is no terminating }, an error occurs.
Characters whose value is less than 256 can be defined by either of the two syntaxes for
\x. There is no difference in the way they are handled. For example, \xdc is exactly the
same as \x{dc}.
Constraints on character values
Characters that are specified using octal or hexadecimal numbers are limited to certain
values, as follows:
8-bit non-UTF mode:
< 0x100
8-bit UTF-8 mode:
< 0x10ffff and a valid codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called "surrogate"
codepoints), and 0xffef.
Escape sequences in character classes
All the sequences that define a single character value can be used both inside and outside
character classes. Also, inside a character class, \b is interpreted as the backspace
character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special inside a character
class. Like other unrecognized escape sequences, they are treated as the literal
characters "B", "R", and "X". Outside a character class, these sequences have different
meanings.
Unsupported Escape Sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to
modify the case of following characters. PCRE does not support these escape sequences.
Absolute and Relative Back References
The sequence \g followed by an unsigned or a negative number, optionally enclosed in
braces, is an absolute or relative back reference. A named back reference can be coded as
\g{name}. Back references are discussed later, following the discussion of parenthesized
subpatterns.
Absolute and Relative Subroutine Calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number
enclosed either in angle brackets or single quotes, is alternative syntax for referencing
a subpattern as a "subroutine". Details are discussed later. Notice that \g{...} (Perl
syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference
and the latter is a subroutine call.
Generic Character Types
Another use of backslash is for specifying generic character types:
\d:
Any decimal digit
\D:
Any character that is not a decimal digit
\h:
Any horizontal whitespace character
\H:
Any character that is not a horizontal whitespace character
\s:
Any whitespace character
\S:
Any character that is not a whitespace character
\v:
Any vertical whitespace character
\V:
Any character that is not a vertical whitespace character
\w:
Any "word" character
\W:
Any "non-word" character
There is also the single sequence \N, which matches a non-newline character. This is the
same as the "." metacharacter when dotall is not set. Perl also uses \N to match
characters by name, but PCRE does not support this.
Each pair of lowercase and uppercase escape sequences partitions the complete set of
characters into two disjoint sets. Any given character matches one, and only one, of each
pair. The sequences can appear both inside and outside character classes. They each match
one character of the appropriate type. If the current matching point is at the end of the
subject string, all fail, as there is no character to match.
For compatibility with Perl, \s did not used to match the VT character (code 11), which
made it different from the the POSIX "space" class. However, Perl added VT at release
5.18, and PCRE followed suit at release 8.34. The default \s characters are now HT (9), LF
(10), VT (11), FF (12), CR (13), and space (32), which are defined as white space in the
"C" locale. This list may vary if locale-specific matching is taking place. For example,
in some locales the "non-breaking space" character (\xA0) is recognized as white space,
and in others the VT character is not.
A "word" character is an underscore or any character that is a letter or a digit. By
default, the definition of letters and digits is controlled by the PCRE low-valued
character tables, in Erlang's case (and without option unicode), the ISO Latin-1 character
set.
By default, in unicode mode, characters with values > 255, that is, all characters outside
the ISO Latin-1 character set, never match \d, \s, or \w, and always match \D, \S, and \W.
These sequences retain their original meanings from before UTF support was available,
mainly for efficiency reasons. However, if option ucp is set, the behavior is changed so
that Unicode properties are used to determine character types, as follows:
\d:
Any character that \p{Nd} matches (decimal digit)
\s:
Any character that \p{Z} or \h or \v
\w:
Any character that matches \p{L} or \p{N} matches, plus underscore
The uppercase escapes match the inverse sets of characters. Notice that \d matches only
decimal digits, while \w matches any Unicode digit, any Unicode letter, and underscore.
Notice also that ucp affects \b and \B, as they are defined in terms of \w and \W.
Matching these sequences is noticeably slower when ucp is set.
The sequences \h, \H, \v, and \V are features that were added to Perl in release 5.10. In
contrast to the other sequences, which match only ASCII characters by default, these
always match certain high-valued code points, regardless if ucp is set.
The following are the horizontal space characters:
U+0009:
Horizontal tab (HT)
U+0020:
Space
U+00A0:
Non-break space
U+1680:
Ogham space mark
U+180E:
Mongolian vowel separator
U+2000:
En quad
U+2001:
Em quad
U+2002:
En space
U+2003:
Em space
U+2004:
Three-per-em space
U+2005:
Four-per-em space
U+2006:
Six-per-em space
U+2007:
Figure space
U+2008:
Punctuation space
U+2009:
Thin space
U+200A:
Hair space
U+202F:
Narrow no-break space
U+205F:
Medium mathematical space
U+3000:
Ideographic space
The following are the vertical space characters:
U+000A:
Line feed (LF)
U+000B:
Vertical tab (VT)
U+000C:
Form feed (FF)
U+000D:
Carriage return (CR)
U+0085:
Next line (NEL)
U+2028:
Line separator
U+2029:
Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with code points < 256 are relevant.
Newline Sequences
Outside a character class, by default, the escape sequence \R matches any Unicode newline
sequence. In non-UTF-8 mode, \R is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details are provided below.
This particular group matches either the two-character sequence CR followed by LF, or one
of the single characters LF (line feed, U+000A), VT (vertical tab, U+000B), FF (form feed,
U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085). The two-character
sequence is treated as a single unit that cannot be split.
In Unicode mode, two more characters whose code points are > 255 are added: LS (line
separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property
support is not needed for these characters to be recognized.
\R can be restricted to match only CR, LF, or CRLF (instead of the complete set of Unicode
line endings) by setting option bsr_anycrlf either at compile time or when the pattern is
matched. (BSR is an acronym for "backslash R".) This can be made the default when PCRE is
built; if so, the other behavior can be requested through option bsr_unicode. These
settings can also be specified by starting a pattern string with one of the following
sequences:
(*BSR_ANYCRLF):
CR, LF, or CRLF only
(*BSR_UNICODE):
Any Unicode newline sequence
These override the default and the options specified to the compiling function, but they
can themselves be overridden by options specified to a matching function. Notice that
these special settings, which are not Perl-compatible, are recognized only at the very
start of a pattern, and that they must be in upper case. If more than one of them is
present, the last one is used. They can be combined with a change of newline convention;
for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF), or (*UCP) special sequences. Inside a
character class, \R is treated as an unrecognized escape sequence, and so matches the
letter "R" by default.
Unicode Character Properties
Three more escape sequences that match characters with specific properties are available.
When in 8-bit non-UTF-8 mode, these sequences are limited to testing characters whose code
points are < 256, but they do work in this mode. The following are the extra escape
sequences:
\p{xx}:
A character with property xx
\P{xx}:
A character without property xx
\X:
A Unicode extended grapheme cluster
The property names represented by xx above are limited to the Unicode script names, the
general category properties, "Any", which matches any character (including newline), and
some special PCRE properties (described in the next section). Other Perl properties, such
as "InMusicalSymbols", are currently not supported by PCRE. Notice that \P{Any} does not
match any characters and always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts. A character from
one of these sets can be matched using a script name, for example:
\p{Greek} \P{Han}
Those that are not part of an identified script are lumped together as "Common". The
following is the current list of scripts:
* Arabic
* Armenian
* Avestan
* Balinese
* Bamum
* Bassa_Vah
* Batak
* Bengali
* Bopomofo
* Braille
* Buginese
* Buhid
* Canadian_Aboriginal
* Carian
* Caucasian_Albanian
* Chakma
* Cham
* Cherokee
* Common
* Coptic
* Cuneiform
* Cypriot
* Cyrillic
* Deseret
* Devanagari
* Duployan
* Egyptian_Hieroglyphs
* Elbasan
* Ethiopic
* Georgian
* Glagolitic
* Gothic
* Grantha
* Greek
* Gujarati
* Gurmukhi
* Han
* Hangul
* Hanunoo
* Hebrew
* Hiragana
* Imperial_Aramaic
* Inherited
* Inscriptional_Pahlavi
* Inscriptional_Parthian
* Javanese
* Kaithi
* Kannada
* Katakana
* Kayah_Li
* Kharoshthi
* Khmer
* Khojki
* Khudawadi
* Lao
* Latin
* Lepcha
* Limbu
* Linear_A
* Linear_B
* Lisu
* Lycian
* Lydian
* Mahajani
* Malayalam
* Mandaic
* Manichaean
* Meetei_Mayek
* Mende_Kikakui
* Meroitic_Cursive
* Meroitic_Hieroglyphs
* Miao
* Modi
* Mongolian
* Mro
* Myanmar
* Nabataean
* New_Tai_Lue
* Nko
* Ogham
* Ol_Chiki
* Old_Italic
* Old_North_Arabian
* Old_Permic
* Old_Persian
* Oriya
* Old_South_Arabian
* Old_Turkic
* Osmanya
* Pahawh_Hmong
* Palmyrene
* Pau_Cin_Hau
* Phags_Pa
* Phoenician
* Psalter_Pahlavi
* Rejang
* Runic
* Samaritan
* Saurashtra
* Sharada
* Shavian
* Siddham
* Sinhala
* Sora_Sompeng
* Sundanese
* Syloti_Nagri
* Syriac
* Tagalog
* Tagbanwa
* Tai_Le
* Tai_Tham
* Tai_Viet
* Takri
* Tamil
* Telugu
* Thaana
* Thai
* Tibetan
* Tifinagh
* Tirhuta
* Ugaritic
* Vai
* Warang_Citi
* Yi
Each character has exactly one Unicode general category property, specified by a two-
letter acronym. For compatibility with Perl, negation can be specified by including a
circumflex between the opening brace and the property name. For example, \p{^Lu} is the
same as \P{Lu}.
If only one letter is specified with \p or \P, it includes all the general category
properties that start with that letter. In this case, in the absence of negation, the
curly brackets in the escape sequence are optional. The following two examples have the
same effect:
\p{L}
\pL
The following general category property codes are supported:
C:
Other
Cc:
Control
Cf:
Format
Cn:
Unassigned
Co:
Private use
Cs:
Surrogate
L:
Letter
Ll:
Lowercase letter
Lm:
Modifier letter
Lo:
Other letter
Lt:
Title case letter
Lu:
Uppercase letter
M:
Mark
Mc:
Spacing mark
Me:
Enclosing mark
Mn:
Non-spacing mark
N:
Number
Nd:
Decimal number
Nl:
Letter number
No:
Other number
P:
Punctuation
Pc:
Connector punctuation
Pd:
Dash punctuation
Pe:
Close punctuation
Pf:
Final punctuation
Pi:
Initial punctuation
Po:
Other punctuation
Ps:
Open punctuation
S:
Symbol
Sc:
Currency symbol
Sk:
Modifier symbol
Sm:
Mathematical symbol
So:
Other symbol
Z:
Separator
Zl:
Line separator
Zp:
Paragraph separator
Zs:
Space separator
The special property L& is also supported. It matches a character that has the Lu, Ll, or
Lt property, that is, a letter that is not classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such
characters are invalid in Unicode strings and so cannot be tested by PCRE. Perl does not
support the Cs property.
The long synonyms for property names supported by Perl (such as \p{Letter}) are not
supported by PCRE. It is not permitted to prefix any of these properties with "Is".
No character in the Unicode table has the Cn (unassigned) property. This property is
instead assumed for any code point that is not in the Unicode table.
Specifying caseless matching does not affect these escape sequences. For example, \p{Lu}
always matches only uppercase letters. This is different from the behavior of current
versions of Perl.
Matching characters by Unicode property is not fast, as PCRE must do a multistage table
lookup to find a character property. That is why the traditional escape sequences such as
\d and \w do not use Unicode properties in PCRE by default. However, you can make them do
so by setting option ucp or by starting the pattern with (*UCP).
Extended Grapheme Clusters
The \X escape matches any number of Unicode characters that form an "extended grapheme
cluster", and treats the sequence as an atomic group (see below). Up to and including
release 8.31, PCRE matched an earlier, simpler definition that was equivalent to
(?>\PM\pM*). That is, it matched a character without the "mark" property, followed by zero
or more characters with the "mark" property. Characters with the "mark" property are
typically non-spacing accents that affect the preceding character.
This simple definition was extended in Unicode to include more complicated kinds of
composite character by giving each character a grapheme breaking property, and creating
rules that use these properties to define the boundaries of extended grapheme clusters. In
PCRE releases later than 8.31, \X matches one of these clusters.
\X always matches at least one character. Then it decides whether to add more characters
according to the following rules for ending a cluster:
* End at the end of the subject string.
* Do not end between CR and LF; otherwise end after any control character.
* Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of
five types: L, V, T, LV, and LVT. An L character can be followed by an L, V, LV, or
LVT character. An LV or V character can be followed by a V or T character. An LVT or T
character can be followed only by a T character.
* Do not end before extending characters or spacing marks. Characters with the "mark"
property always have the "extend" grapheme breaking property.
* Do not end after prepend characters.
* Otherwise, end the cluster.
PCRE Additional Properties
In addition to the standard Unicode properties described earlier, PCRE supports four more
that make it possible to convert traditional escape sequences, such as \w and \s to use
Unicode properties. PCRE uses these non-standard, non-Perl properties internally when the
ucp option is passed. However, they can also be used explicitly. The properties are as
follows:
Xan:
Any alphanumeric character. Matches characters that have either the L (letter) or the
N (number) property.
Xps:
Any Posix space character. Matches the characters tab, line feed, vertical tab, form
feed, carriage return, and any other character that has the Z (separator) property.
Xsp:
Any Perl space character. Matches the same as Xps, except that vertical tab is
excluded.
Xwd:
Any Perl "word" character. Matches the same characters as Xan, plus underscore.
Perl and POSIX space are now the same. Perl added VT to its space character set at release
5.18 and PCRE changed at release 8.34.
Xan matches characters that have either the L (letter) or the N (number) property. Xps
matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any
other character that has the Z (separator) property. Xsp is the same as Xps; it used to
exclude vertical tab, for Perl compatibility, but Perl changed, and so PCRE followed at
release 8.34. Xwd matches the same characters as Xan, plus underscore.
There is another non-standard property, Xuc, which matches any character that can be
represented by a Universal Character Name in C++ and other programming languages. These
are the characters $, @, ` (grave accent), and all characters with Unicode code points >=
U+00A0, except for the surrogates U+D800 to U+DFFF. Notice that most base (ASCII)
characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH,
where H is a hexadecimal digit. Notice that the Xuc property does not match these
sequences but the characters that they represent.)
Resetting the Match Start
The escape sequence \K causes any previously matched characters not to be included in the
final matched sequence. For example, the following pattern matches "foobar", but reports
that it has matched "bar":
foo\Kbar
This feature is similar to a lookbehind assertion (described below). However, in this
case, the part of the subject before the real match does not have to be of fixed length,
as lookbehind assertions do. The use of \K does not interfere with the setting of captured
substrings. For example, when the following pattern matches "foobar", the first substring
is still set to "foo":
(foo)\Kbar
Perl documents that the use of \K within assertions is "not well defined". In PCRE, \K is
acted upon when it occurs inside positive assertions, but is ignored in negative
assertions. Note that when a pattern such as (?=ab\K) matches, the reported start of the
match can be greater than the end of the match.
Simple Assertions
The final use of backslash is for certain simple assertions. An assertion specifies a
condition that must be met at a particular point in a match, without consuming any
characters from the subject string. The use of subpatterns for more complicated assertions
is described below. The following are the backslashed assertions:
\b:
Matches at a word boundary.
\B:
Matches when not at a word boundary.
\A:
Matches at the start of the subject.
\Z:
Matches at the end of the subject, and before a newline at the end of the subject.
\z:
Matches only at the end of the subject.
\G:
Matches at the first matching position in the subject.
Inside a character class, \b has a different meaning; it matches the backspace character.
If any other of these assertions appears in a character class, by default it matches the
corresponding literal character (for example, \B matches the letter B).
A word boundary is a position in the subject string where the current character and the
previous character do not both match \w or \W (that is, one matches \w and the other
matches \W), or the start or end of the string if the first or last character matches \w,
respectively. In UTF mode, the meanings of \w and \W can be changed by setting option ucp.
When this is done, it also affects \b and \B. PCRE and Perl do not have a separate "start
of word" or "end of word" metasequence. However, whatever follows \b normally determines
which it is. For example, the fragment \ba matches "a" at the start of a word.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described
in the next section) in that they only ever match at the very start and end of the subject
string, whatever options are set. Thus, they are independent of multiline mode. These
three assertions are not affected by options notbol or noteol, which affect only the
behavior of the circumflex and dollar metacharacters. However, if argument startoffset of
run/3 is non-zero, indicating that matching is to start at a point other than the
beginning of the subject, \A can never match. The difference between \Z and \z is that \Z
matches before a newline at the end of the string and at the very end, while \z matches
only at the end.
The \G assertion is true only when the current matching position is at the start point of
the match, as specified by argument startoffset of run/3. It differs from \A when the
value of startoffset is non-zero. By calling run/3 multiple times with appropriate
arguments, you can mimic the Perl option /g, and it is in this kind of implementation
where \G can be useful.
Notice, however, that the PCRE interpretation of \G, as the start of the current match, is
subtly different from Perl, which defines it as the end of the previous match. In Perl,
these can be different when the previously matched string was empty. As PCRE does only one
match at a time, it cannot reproduce this behavior.
If all the alternatives of a pattern begin with \G, the expression is anchored to the
starting match position, and the "anchored" flag is set in the compiled regular
expression.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width assertions. That is, they test for
a particular condition to be true without consuming any characters from the subject
string.
Outside a character class, in the default matching mode, the circumflex character is an
assertion that is true only if the current matching point is at the start of the subject
string. If argument startoffset of run/3 is non-zero, circumflex can never match if option
multiline is unset. Inside a character class, circumflex has an entirely different meaning
(see below).
Circumflex needs not to be the first character of the pattern if some alternatives are
involved, but it is to be the first thing in each alternative in which it appears if the
pattern is ever to match that branch. If all possible alternatives start with a
circumflex, that is, if the pattern is constrained to match only at the start of the
subject, it is said to be an "anchored" pattern. (There are also other constructs that can
cause a pattern to be anchored.)
The dollar character is an assertion that is true only if the current matching point is at
the end of the subject string, or immediately before a newline at the end of the string
(by default). Notice however that it does not match the newline. Dollar needs not to be
the last character of the pattern if some alternatives are involved, but it is to be the
last item in any branch in which it appears. Dollar has no special meaning in a character
class.
The meaning of dollar can be changed so that it matches only at the very end of the
string, by setting option dollar_endonly at compile time. This does not affect the \Z
assertion.
The meanings of the circumflex and dollar characters are changed if option multiline is
set. When this is the case, a circumflex matches immediately after internal newlines and
at the start of the subject string. It does not match after a newline that ends the
string. A dollar matches before any newlines in the string, and at the very end, when
multiline is set. When newline is specified as the two-character sequence CRLF, isolated
CR and LF characters do not indicate newlines.
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n
represents a newline) in multiline mode, but not otherwise. So, patterns that are anchored
in single-line mode because all branches start with ^ are not anchored in multiline mode,
and a match for circumflex is possible when argument startoffset of run/3 is non-zero.
Option dollar_endonly is ignored if multiline is set.
Notice that the sequences \A, \Z, and \z can be used to match the start and end of the
subject in both modes. If all branches of a pattern start with \A, it is always anchored,
regardless if multiline is set.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any character in the subject
string except (by default) a character that signifies the end of a line.
When a line ending is defined as a single character, dot never matches that character.
When the two-character sequence CRLF is used, dot does not match CR if it is immediately
followed by LF, otherwise it matches all characters (including isolated CRs and LFs). When
any Unicode line endings are recognized, dot does not match CR, LF, or any of the other
line-ending characters.
The behavior of dot regarding newlines can be changed. If option dotall is set, a dot
matches any character, without exception. If the two-character sequence CRLF is present in
the subject string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circumflex and dollar, the
only relationship is that both involve newlines. Dot has no special meaning in a character
class.
The escape sequence \N behaves like a dot, except that it is not affected by option
PCRE_DOTALL. That is, it matches any character except one that signifies the end of a
line. Perl also uses \N to match characters by name but PCRE does not support this.
MATCHING A SINGLE DATA UNIT
Outside a character class, the escape sequence \C matches any data unit, regardless if a
UTF mode is set. One data unit is one byte. Unlike a dot, \C always matches line-ending
characters. The feature is provided in Perl to match individual bytes in UTF-8 mode, but
it is unclear how it can usefully be used. As \C breaks up characters into individual data
units, matching one unit with \C in a UTF mode means that the remaining string can start
with a malformed UTF character. This has undefined results, as PCRE assumes that it deals
with valid UTF strings.
PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode,
as this would make it impossible to calculate the length of the lookbehind.
The \C escape sequence is best avoided. However, one way of using it that avoids the
problem of malformed UTF characters is to use a lookahead to check the length of the next
character, as in the following pattern, which can be used with a UTF-8 string (ignore
whitespace and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing parentheses numbers in each alternative
(see section Duplicate Subpattern Numbers). The assertions at the start of each branch
check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes,
respectively. The individual bytes of the character are then captured by the appropriate
number of groups.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a closing square
bracket. A closing square bracket on its own is not special by default. However, if option
PCRE_JAVASCRIPT_COMPAT is set, a lone closing square bracket causes a compile-time error.
If a closing square bracket is required as a member of the class, it is to be the first
data character in the class (after an initial circumflex, if present) or escaped with a
backslash.
A character class matches a single character in the subject. In a UTF mode, the character
can be more than one data unit long. A matched character must be in the set of characters
defined by the class, unless the first character in the class definition is a circumflex,
in which case the subject character must not be in the set defined by the class. If a
circumflex is required as a member of the class, ensure that it is not the first
character, or escape it with a backslash.
For example, the character class [aeiou] matches any lowercase vowel, while [^aeiou]
matches any character that is not a lowercase vowel. Notice that a circumflex is just a
convenient notation for specifying the characters that are in the class by enumerating
those that are not. A class that starts with a circumflex is not an assertion; it still
consumes a character from the subject string, and therefore it fails if the current
pointer is at the end of the string.
In UTF-8 mode, characters with values > 255 (0xffff) can be included in a class as a
literal string of data units, or by using the \x{ escaping mechanism.
When caseless matching is set, any letters in a class represent both their uppercase and
lowercase versions. For example, a caseless [aeiou] matches "A" and "a", and a caseless
[^aeiou] does not match "A", but a caseful version would. In a UTF mode, PCRE always
understands the concept of case for characters whose values are < 256, so caseless
matching is always possible. For characters with higher values, the concept of case is
supported only if PCRE is compiled with Unicode property support. If you want to use
caseless matching in a UTF mode for characters >=, ensure that PCRE is compiled with
Unicode property support and with UTF support.
Characters that can indicate line breaks are never treated in any special way when
matching character classes, whatever line-ending sequence is in use, and whatever setting
of options PCRE_DOTALL and PCRE_MULTILINE is used. A class such as [^a] always matches one
of these characters.
The minus (hyphen) character can be used to specify a range of characters in a character
class. For example, [d-m] matches any letter between d and m, inclusive. If a minus
character is required in a class, it must be escaped with a backslash or appear in a
position where it cannot be interpreted as indicating a range, typically as the first or
last character in the class, or immediately after a range. For example, [b-d-z] matches
letters in the range b to d, a hyphen character, or z.
The literal character "]" cannot be the end character of a range. A pattern such as
[W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal
string "46]", so it would match "W46]" or "-46]". However, if "]" is escaped with a
backslash, it is interpreted as the end of range, so [W-\]46] is interpreted as a class
containing a range followed by two other characters. The octal or hexadecimal
representation of "]" can also be used to end a range.
An error is generated if a POSIX character class (see below) or an escape sequence other
than one that defines a single character appears at a point where a range ending character
is expected. For example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.
Ranges operate in the collating sequence of character values. They can also be used for
characters specified numerically, for example, [\000-\037]. Ranges can include any
characters that are valid for the current mode.
If a range that includes letters is used when caseless matching is set, it matches the
letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
caselessly. In a non-UTF mode, if character tables for a French locale are in use,
[\xc8-\xcb] matches accented E characters in both cases. In UTF modes, PCRE supports the
concept of case for characters with values > 255 only when it is compiled with Unicode
property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W can
appear in a character class, and add the characters that they match to the class. For
example, [\dABCDEF] matches any hexadecimal digit. In UTF modes, option ucp affects the
meanings of \d, \s, \w and their uppercase partners, just as it does when they appear
outside a character class, as described in section Generic Character Types earlier. The
escape sequence \b has a different meaning inside a character class; it matches the
backspace character. The sequences \B, \N, \R, and \X are not special inside a character
class. Like any other unrecognized escape sequences, they are treated as the literal
characters "B", "N", "R", and "X".
A circumflex can conveniently be used with the uppercase character types to specify a more
restricted set of characters than the matching lowercase type. For example, class [^\W_]
matches any letter or digit, but not underscore, while [\w] includes underscore. A
positive character class is to be read as "something OR something OR ..." and a negative
class as "NOT something AND NOT something AND NOT ...".
Only the following metacharacters are recognized in character classes:
* Backslash
* Hyphen (only where it can be interpreted as specifying a range)
* Circumflex (only at the start)
* Opening square bracket (only when it can be interpreted as introducing a Posix class
name, or for a special compatibility feature; see the next two sections)
* Terminating closing square bracket
However, escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the Posix notation for character classes. This uses names enclosed by [: and
:] within the enclosing square brackets. PCRE also supports this notation. For example,
the following matches "0", "1", any alphabetic character, or "%":
[01[:alpha:]%]
The following are the supported class names:
alnum:
Letters and digits
alpha:
Letters
ascii:
Character codes 0-127
blank:
Space or tab only
cntrl:
Control characters
digit:
Decimal digits (same as \d)
graph:
Printing characters, excluding space
lower:
Lowercase letters
print:
Printing characters, including space
punct:
Printing characters, excluding letters, digits, and space
space:
Whitespace (the same as \s from PCRE 8.34)
upper:
Uppercase letters
word:
"Word" characters (same as \w)
xdigit:
Hexadecimal digits
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space
(32). If locale-specific matching is taking place, the list of space characters may be
different; there may be fewer or more of them. "Space" used to be different to \s, which
did not include VT, for Perl compatibility. However, Perl changed at release 5.18, and
PCRE followed at release 8.34. "Space" and \s now match the same set of characters.
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another
Perl extension is negation, which is indicated by a ^ character after the colon. For
example, the following matches "1", "2", or any non-digit:
[12[:^digit:]]
PCRE (and Perl) also recognize the Posix syntax [.ch.] and [=ch=] where "ch" is a
"collating element", but these are not supported, and an error is given if they are
encountered.
By default, characters with values > 255 do not match any of the Posix character classes.
However, if option PCRE_UCP is passed to pcre_compile(), some of the classes are changed
so that Unicode character properties are used. This is achieved by replacing certain Posix
classes by other sequences, as follows:
[:alnum:]:
Becomes \p{Xan}
[:alpha:]:
Becomes \p{L}
[:blank:]:
Becomes \h
[:digit:]:
Becomes \p{Nd}
[:lower:]:
Becomes \p{Ll}
[:space:]:
Becomes \p{Xps}
[:upper:]:
Becomes \p{Lu}
[:word:]:
Becomes \p{Xwd}
Negated versions, such as [:^alpha:], use \P instead of \p. Three other POSIX classes are
handled specially in UCP mode:
[:graph:]:
This matches characters that have glyphs that mark the page when printed. In Unicode
property terms, it matches all characters with the L, M, N, P, S, or Cf properties,
except for:
U+061C:
Arabic Letter Mark
U+180E:
Mongolian Vowel Separator
U+2066 - U+2069:
Various "isolate"s
[:print:]:
This matches the same characters as [:graph:] plus space characters that are not
controls, that is, characters with the Zs property.
[:punct:]:
This matches all characters that have the Unicode P (punctuation) property, plus those
characters whose code points are less than 128 that have the S (Symbol) property.
The other POSIX classes are unchanged, and match only characters with code points less
than 128.
Compatibility Feature for Word Boundaries
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly syntax [[:<:]]
and [[:>:]] is used for matching "start of word" and "end of word". PCRE treats these
items as follows:
[[:<:]]:
is converted to \b(?=\w)
[[:>:]]:
is converted to \b(?<=\w)
Only these exact character sequences are recognized. A sequence such as [a[:<:]b] provokes
error for an unrecognized POSIX class name. This support is not compatible with Perl. It
is provided to help migrations from other environments, and is best not used in any new
patterns. Note that \b matches at the start and the end of a word (see "Simple assertions"
above), and in a Perl-style pattern the preceding or following character normally shows
which is wanted, without the need for the assertions that are used above in order to give
exactly the POSIX behaviour.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example, the
following pattern matches either "gilbert" or "sullivan":
gilbert|sullivan
Any number of alternatives can appear, and an empty alternative is permitted (matching the
empty string). The matching process tries each alternative in turn, from left to right,
and the first that succeeds is used. If the alternatives are within a subpattern (defined
in section Subpatterns), "succeeds" means matching the remaining main pattern and the
alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the Perl-compatible options caseless, multiline, dotall, and extended can
be changed from within the pattern by a sequence of Perl option letters enclosed between
"(?" and ")". The option letters are as follows:
i:
For caseless
m:
For multiline
s:
For dotall
x:
For extended
For example, (?im) sets caseless, multiline matching. These options can also be unset by
preceding the letter with a hyphen. A combined setting and unsetting such as (?im-sx),
which sets caseless and multiline, while unsetting dotall and extended, is also permitted.
If a letter appears both before and after the hyphen, the option is unset.
The PCRE-specific options dupnames, ungreedy, and extra can be changed in the same way as
the Perl-compatible options by using the characters J, U, and X respectively.
When one of these option changes occurs at top-level (that is, not inside subpattern
parentheses), the change applies to the remainder of the pattern that follows.
An option change within a subpattern (see section Subpatterns) affects only that part of
the subpattern that follows it. So, the following matches abc and aBc and no other strings
(assuming caseless is not used):
(a(?i)b)c
By this means, options can be made to have different settings in different parts of the
pattern. Any changes made in one alternative do carry on into subsequent branches within
the same subpattern. For example:
(a(?i)b|c)
matches "ab", "aB", "c", and "C", although when matching "C" the first branch is abandoned
before the option setting. This is because the effects of option settings occur at compile
time. There would be some weird behavior otherwise.
Note:
Other PCRE-specific options can be set by the application when the compiling or matching
functions are called. Sometimes the pattern can contain special leading sequences, such as
(*CRLF), to override what the application has set or what has been defaulted. Details are
provided in section Newline Sequences earlier.
The (*UTF8) and (*UCP) leading sequences can be used to set UTF and Unicode property
modes. They are equivalent to setting options unicode and ucp, respectively. The (*UTF)
sequence is a generic version that can be used with any of the libraries. However, the
application can set option never_utf, which locks out the use of the (*UTF) sequences.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning
part of a pattern into a subpattern does two things:
1.:
It localizes a set of alternatives. For example, the following pattern matches
"cataract", "caterpillar", or "cat":
cat(aract|erpillar|)
Without the parentheses, it would match "cataract", "erpillar", or an empty string.
2.:
It sets up the subpattern as a capturing subpattern. That is, when the complete
pattern matches, that portion of the subject string that matched the subpattern is
passed back to the caller through the return value of run/3.
Opening parentheses are counted from left to right (starting from 1) to obtain numbers for
the capturing subpatterns. For example, if the string "the red king" is matched against
the following pattern, the captured substrings are "red king", "red", and "king", and are
numbered 1, 2, and 3, respectively:
the ((red|white) (king|queen))
It is not always helpful that plain parentheses fulfill two functions. Often a grouping
subpattern is required without a capturing requirement. If an opening parenthesis is
followed by a question mark and a colon, the subpattern does not do any capturing, and is
not counted when computing the number of any subsequent capturing subpatterns. For
example, if the string "the white queen" is matched against the following pattern, the
captured substrings are "white queen" and "queen", and are numbered 1 and 2:
the ((?:red|white) (king|queen))
The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the start of a non-
capturing subpattern, the option letters can appear between "?" and ":". Thus, the
following two patterns match the same set of strings:
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
As alternative branches are tried from left to right, and options are not reset until the
end of the subpattern is reached, an option setting in one branch does affect subsequent
branches, so the above patterns match both "SUNDAY" and "Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature where each alternative in a subpattern uses the same
numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a
non-capturing subpattern. For example, consider the following pattern:
(?|(Sat)ur|(Sun))day
As the two alternatives are inside a (?| group, both sets of capturing parentheses are
numbered one. Thus, when the pattern matches, you can look at captured substring number
one, whichever alternative matched. This construct is useful when you want to capture a
part, but not all, of one of many alternatives. Inside a (?| group, parentheses are
numbered as usual, but the number is reset at the start of each branch. The numbers of any
capturing parentheses that follow the subpattern start after the highest number used in
any branch. The following example is from the Perl documentation; the numbers underneath
show in which buffer the captured content is stored:
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most recent value that is set for that
number by any subpattern. The following pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always refers to the first one in
the pattern with the given number. The following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern having matched refers to a non-unique number, the
test is true if any of the subpatterns of that number have matched.
An alternative approach using this "branch reset" feature is to use duplicate named
subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be hard to keep track of
the numbers in complicated regular expressions. Also, if an expression is modified, the
numbers can change. To help with this difficulty, PCRE supports the naming of subpatterns.
This feature was not added to Perl until release 5.10. Python had the feature earlier, and
PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the
Perl and the Python syntax. Perl allows identically numbered subpatterns to have different
names, but PCRE does not.
In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as
in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other
parts of the pattern, such as back references, recursion, and conditions, can be made by
name and by number.
Names consist of up to 32 alphanumeric characters and underscores, but must start with a
non-digit. Named capturing parentheses are still allocated numbers as well as names,
exactly as if the names were not present. The capture specification to run/3 can use named
values if they are present in the regular expression.
By default, a name must be unique within a pattern, but this constraint can be relaxed by
setting option dupnames at compile time. (Duplicate names are also always permitted for
subpatterns with the same number, set up as described in the previous section.) Duplicate
names can be useful for patterns where only one instance of the named parentheses can
match. Suppose that you want to match the name of a weekday, either as a 3-letter
abbreviation or as the full name, and in both cases you want to extract the abbreviation.
The following pattern (ignoring the line breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, but only one is ever set after a match. (An
alternative way of solving this problem is to use a "branch reset" subpattern, as
described in the previous section.)
For capturing named subpatterns which names are not unique, the first matching occurrence
(counted from left to right in the subject) is returned from run/3, if the name is
specified in the values part of the capture statement. The all_names capturing value
matches all the names in the same way.
Note:
You cannot use different names to distinguish between two subpatterns with the same
number, as PCRE uses only the numbers when matching. For this reason, an error is given at
compile time if different names are specified to subpatterns with the same number.
However, you can specify the same name to subpatterns with the same number, even when
dupnames is not set.
REPETITION
Repetition is specified by quantifiers, which can follow any of the following items:
* A literal data character
* The dot metacharacter
* The \C escape sequence
* The \X escape sequence
* The \R escape sequence
* An escape such as \d or \pL that matches a single character
* A character class
* A back reference (see the next section)
* A parenthesized subpattern (including assertions)
* A subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum number of permitted
matches, by giving the two numbers in curly brackets (braces), separated by a comma. The
numbers must be < 65536, and the first must be less than or equal to the second. For
example, the following matches "zz", "zzz", or "zzzz":
z{2,4}
A closing brace on its own is not a special character. If the second number is omitted,
but the comma is present, there is no upper limit. If the second number and the comma are
both omitted, the quantifier specifies an exact number of required matches. Thus, the
following matches at least three successive vowels, but can match many more:
[aeiou]{3,}
The following matches exactly eight digits:
\d{8}
An opening curly bracket that appears in a position where a quantifier is not allowed, or
one that does not match the syntax of a quantifier, is taken as a literal character. For
example, {,6} is not a quantifier, but a literal string of four characters.
In Unicode mode, quantifiers apply to characters rather than to individual data units.
Thus, for example, \x{100}{2} matches two characters, each of which is represented by a
2-byte sequence in a UTF-8 string. Similarly, \X{3} matches three Unicode extended
grapheme clusters, each of which can be many data units long (and they can be of different
lengths).
The quantifier {0} is permitted, causing the expression to behave as if the previous item
and the quantifier were not present. This can be useful for subpatterns that are
referenced as subroutines from elsewhere in the pattern (but see also section Defining
Subpatterns for Use by Reference Only). Items other than subpatterns that have a {0}
quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-character abbreviations:
*:
Equivalent to {0,}
+:
Equivalent to {1,}
?:
Equivalent to {0,1}
Infinite loops can be constructed by following a subpattern that can match no characters
with a quantifier that has no upper limit, for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time for such patterns.
However, as there are cases where this can be useful, such patterns are now accepted.
However, if any repetition of the subpattern matches no characters, the loop is forcibly
broken.
By default, the quantifiers are "greedy", that is, they match as much as possible (up to
the maximum number of permitted times), without causing the remaining pattern to fail. The
classic example of where this gives problems is in trying to match comments in C programs.
These appear between /* and */. Within the comment, individual * and / characters can
appear. An attempt to match C comments by applying the pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, as it matches the entire string owing to the greediness of the .* item.
However, if a quantifier is followed by a question mark, it ceases to be greedy, and
instead matches the minimum number of times possible, so the following pattern does the
right thing with the C comments:
/\*.*?\*/
The meaning of the various quantifiers is not otherwise changed, only the preferred number
of matches. Do not confuse this use of question mark with its use as a quantifier in its
own right. As it has two uses, it can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the only way the
remaining pattern matches.
If option ungreedy is set (an option that is not available in Perl), the quantifiers are
not greedy by default, but individual ones can be made greedy by following them with a
question mark. That is, it inverts the default behavior.
When a parenthesized subpattern is quantified with a minimum repeat count that is > 1 or
with a limited maximum, more memory is required for the compiled pattern, in proportion to
the size of the minimum or maximum.
If a pattern starts with .* or .{0,} and option dotall (equivalent to Perl option /s) is
set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because
whatever follows is tried against every character position in the subject string. So,
there is no point in retrying the overall match at any position after the first. PCRE
normally treats such a pattern as if it was preceded by \A.
In cases where it is known that the subject string contains no newlines, it is worth
setting dotall to obtain this optimization, or alternatively using ^ to indicate anchoring
explicitly.
However, there are some cases where the optimization cannot be used. When .* is inside
capturing parentheses that are the subject of a back reference elsewhere in the pattern, a
match at the start can fail where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123", the match point is the fourth character. Therefore, such
a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the leading .* is inside an
atomic group. Once again, a match at the start can fail where a later one succeeds.
Consider the following pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking control verbs (*PRUNE)
and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the substring that matched
the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee", the value of the captured substring is "tweedledee".
However, if there are nested capturing subpatterns, the corresponding captured values can
have been set in previous iterations. For example, after
/(a|(b))+/
matches "aba", the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure
of what follows normally causes the repeated item to be re-evaluated to see if a different
number of repeats allows the remaining pattern to match. Sometimes it is useful to prevent
this, either to change the nature of the match, or to cause it to fail earlier than it
otherwise might, when the author of the pattern knows that there is no point in carrying
on.
Consider, for example, the pattern \d+foo when applied to the following subject line:
123456bar
After matching all six digits and then failing to match "foo", the normal action of the
matcher is to try again with only five digits matching item \d+, and then with four, and
so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's
book) provides the means for specifying that once a subpattern has matched, it is not to
be re-evaluated in this way.
If atomic grouping is used for the previous example, the matcher gives up immediately on
failing to match "foo" the first time. The notation is a kind of special parenthesis,
starting with (?> as in the following example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it contains once it has
matched, and a failure further into the pattern is prevented from backtracking into it.
Backtracking past it to previous items, however, works as normal.
An alternative description is that a subpattern of this type matches the string of
characters that an identical standalone pattern would match, if anchored at the current
point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above
example can be thought of as a maximizing repeat that must swallow everything it can. So,
while both \d+ and \d+? are prepared to adjust the number of digits they match to make the
remaining pattern match, (?>\d+) can only match an entire sequence of digits.
Atomic groups in general can contain any complicated subpatterns, and can be nested.
However, when the subpattern for an atomic group is just a single repeated item, as in the
example above, a simpler notation, called a "possessive quantifier" can be used. This
consists of an extra + character following a quantifier. Using this notation, the previous
example can be rewritten as
\d++foo
Notice that a possessive quantifier can be used with an entire group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of option ungreedy is ignored. They
are a convenient notation for the simpler forms of an atomic group. However, there is no
difference in the meaning of a possessive quantifier and the equivalent atomic group, but
there can be a performance difference; possessive quantifiers are probably slightly
faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl
originated the idea (and the name) in the first edition of his book. Mike McCloskey liked
it, so implemented it when he built the Sun Java package, and PCRE copied it from there.
It ultimately found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain simple pattern
constructs. For example, the sequence A+B is treated as A++B, as there is no point in
backtracking into a sequence of A:s when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that can itself be
repeated an unlimited number of times, the use of an atomic group is the only way to avoid
some failing matches taking a long time. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-digits, or digits
enclosed in <>, followed by ! or ?. When it matches, it runs quickly. However, if it is
applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the string can be divided
between the internal \D+ repeat and the external * repeat in many ways, and all must be
tried. (The example uses [!?] rather than a single character at the end, as both PCRE and
Perl have an optimization that allows for fast failure when a single character is used.
They remember the last single character that is required for a match, and fail early if it
is not present in the string.) If the pattern is changed so that it uses an atomic group,
like the following, sequences of non-digits cannot be broken, and failure happens quickly:
((?>\D+)|<\d+>)*[!?]
BACK REFERENCES
Outside a character class, a backslash followed by a digit > 0 (and possibly further
digits) is a back reference to a capturing subpattern earlier (that is, to its left) in
the pattern, provided there have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is < 10, it is always taken as a
back reference, and causes an error only if there are not that many capturing left
parentheses in the entire pattern. That is, the parentheses that are referenced do need
not be to the left of the reference for numbers < 10. A "forward back reference" of this
type can make sense when a repetition is involved and the subpattern to the right has
participated in an earlier iteration.
It is not possible to have a numerical "forward back reference" to a subpattern whose
number is 10 or more using this syntax, as a sequence such as \50 is interpreted as a
character defined in octal. For more details of the handling of digits following a
backslash, see section Non-Printing Characters earlier. There is no such problem when
named parentheses are used. A back reference to any subpattern is possible using named
parentheses (see below).
Another way to avoid the ambiguity inherent in the use of digits following a backslash is
to use the \g escape sequence. This escape must be followed by an unsigned number or a
negative number, optionally enclosed in braces. The following examples are identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the ambiguity that is present
in the older syntax. It is also useful when literal digits follow the reference. A
negative number is a relative reference. Consider the following example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started capturing subpattern
before \g, that is, it is equivalent to \2 in this example. Similarly, \g{-2} would be
equivalent to \1. The use of relative references can be helpful in long patterns, and also
in patterns that are created by joining fragments containing references within themselves.
A back reference matches whatever matched the capturing subpattern in the current subject
string, rather than anything matching the subpattern itself (section Subpattern as
Subroutines describes a way of doing that). So, the following pattern matches "sense and
sensibility" and "response and responsibility", but not "sense and responsibility":
(sens|respons)e and \1ibility
If caseful matching is in force at the time of the back reference, the case of letters is
relevant. For example, the following matches "rah rah" and "RAH RAH", but not "RAH rah",
although the original capturing subpattern is matched caselessly:
((?i)rah)\s+\1
There are many different ways of writing back references to named subpatterns. The .NET
syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python
syntax (?P=name). The unified back reference syntax in Perl 5.10, in which \g can be used
for both numeric and named references, is also supported. The previous example can be
rewritten in the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name can appear in the pattern before or after the
reference.
There can be more than one back reference to the same subpattern. If a subpattern has not
been used in a particular match, any back references to it always fails. For example, the
following pattern always fails if it starts to match "a" rather than "bc":
(a|(bc))\2
As there can be many capturing parentheses in a pattern, all digits following the
backslash are taken as part of a potential back reference number. If the pattern continues
with a digit character, some delimiter must be used to terminate the back reference. If
option extended is set, this can be whitespace. Otherwise an empty comment (see section
Comments) can be used.
Recursive Back References
A back reference that occurs inside the parentheses to which it refers fails when the
subpattern is first used, so, for example, (a\1) never matches. However, such references
can be useful inside repeated subpatterns. For example, the following pattern matches any
number of "a"s and also "aba", "ababbaa", and so on:
(a|b\1)+
At each iteration of the subpattern, the back reference matches the character string
corresponding to the previous iteration. In order for this to work, the pattern must be
such that the first iteration does not need to match the back reference. This can be done
using alternation, as in the example above, or by a quantifier with a minimum of zero.
Back references of this type cause the group that they reference to be treated as an
atomic group. Once the whole group has been matched, a subsequent matching failure cannot
cause backtracking into the middle of the group.
ASSERTIONS
An assertion is a test on the characters following or preceding the current matching point
that does not consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z,
\z, ^, and $ are described in the previous sections.
More complicated assertions are coded as subpatterns. There are two kinds: those that look
ahead of the current position in the subject string, and those that look behind it. An
assertion subpattern is matched in the normal way, except that it does not cause the
current matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such an assertion contains
capturing subpatterns within it, these are counted for the purposes of numbering the
capturing subpatterns in the whole pattern. However, substring capturing is done only for
positive assertions. (Perl sometimes, but not always, performs capturing in negative
assertions.)
Warning:
If a positive assertion containing one or more capturing subpatterns succeeds, but failure
to match later in the pattern causes backtracking over this assertion, the captures within
the assertion are reset only if no higher numbered captures are already set. This is,
unfortunately, a fundamental limitation of the current implementation, and as PCRE1 is now
in maintenance-only status, it is unlikely ever to change.
For compatibility with Perl, assertion subpatterns can be repeated. However, it makes no
sense to assert the same thing many times, the side effect of capturing parentheses can
occasionally be useful. In practice, there are only three cases:
* If the quantifier is {0}, the assertion is never obeyed during matching. However, it
can contain internal capturing parenthesized groups that are called from elsewhere
through the subroutine mechanism.
* If quantifier is {0,n}, where n > 0, it is treated as if it was {0,1}. At runtime, the
remaining pattern match is tried with and without the assertion, the order depends on
the greediness of the quantifier.
* If the minimum repetition is > 0, the quantifier is ignored. The assertion is obeyed
only once when encountered during matching.
Lookahead Assertions
Lookahead assertions start with (?= for positive assertions and (?! for negative
assertions. For example, the following matches a word followed by a semicolon, but does
not include the semicolon in the match:
\w+(?=;)
The following matches any occurrence of "foo" that is not followed by "bar":
foo(?!bar)
Notice that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something other than "foo". It
finds any occurrence of "bar" whatsoever, as the assertion (?!foo) is always true when the
next three characters are "bar". A lookbehind assertion is needed to achieve the other
effect.
If you want to force a matching failure at some point in a pattern, the most convenient
way to do it is with (?!), as an empty string always matches. So, an assertion that
requires there is not to be an empty string must always fail. The backtracking control
verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind Assertions
Lookbehind assertions start with (?<= for positive assertions and (?<! for negative
assertions. For example, the following finds an occurrence of "bar" that is not preceded
by "foo":
(?<!foo)bar
The contents of a lookbehind assertion are restricted such that all the strings it matches
must have a fixed length. However, if there are many top-level alternatives, they do not
all have to have the same fixed length. Thus, the following is permitted:
(?<=bullock|donkey)
The following causes an error at compile time:
(?<!dogs?|cats?)
Branches that match different length strings are permitted only at the top-level of a
lookbehind assertion. This is an extension compared with Perl, which requires all branches
to match the same length of string. An assertion such as the following is not permitted,
as its single top-level branch can match two different lengths:
(?<=ab(c|de))
However, it is acceptable to PCRE if rewritten to use two top-level branches:
(?<=abc|abde)
Sometimes the escape sequence \K (see above) can be used instead of a lookbehind assertion
to get round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative, to move the current
position back temporarily by the fixed length and then try to match. If there are
insufficient characters before the current position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a single data unit even in
a UTF mode) to appear in lookbehind assertions, as it makes it impossible to calculate the
length of the lookbehind. The \X and \R escapes, which can match different numbers of data
units, are not permitted either.
"Subroutine" calls (see below), such as (?2) or (?&X), are permitted in lookbehinds, as
long as the subpattern matches a fixed-length string. Recursion, however, is not
supported.
Possessive quantifiers can be used with lookbehind assertions to specify efficient
matching of fixed-length strings at the end of subject strings. Consider the following
simple pattern when applied to a long string that does not match:
abcd$
As matching proceeds from left to right, PCRE looks for each "a" in the subject and then
sees if what follows matches the remaining pattern. If the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first. However, when this fails (as there is
no following "a"), it backtracks to match all but the last character, then all but the
last two characters, and so on. Once again the search for "a" covers the entire string,
from right to left, so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the entire string. The
subsequent lookbehind assertion does a single test on the last four characters. If it
fails, the match fails immediately. For long strings, this approach makes a significant
difference to the processing time.
Using Multiple Assertions
Many assertions (of any sort) can occur in succession. For example, the following matches
"foo" preceded by three digits that are not "999":
(?<=\d{3})(?<!999)foo
Notice that each of the assertions is applied independently at the same point in the
subject string. First there is a check that the previous three characters are all digits,
and then there is a check that the same three characters are not "999". This pattern does
not match "foo" preceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it does not match "123abcfoo". A pattern to do
that is the following:
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters, checks that the first
three are digits, and then the second assertion checks that the preceding three characters
are not "999".
Assertions can be nested in any combination. For example, the following matches an
occurrence of "baz" that is preceded by "bar", which in turn is not preceded by "foo":
(?<=(?<!foo)bar)baz
The following pattern matches "foo" preceded by three digits and any three characters that
are not "999":
(?<=\d{3}(?!999)...)foo
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern conditionally or to
choose between two alternative subpatterns, depending on the result of an assertion, or
whether a specific capturing subpattern has already been matched. The following are the
two possible forms of conditional subpattern:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used, otherwise the no-pattern (if
present). If more than two alternatives exist in the subpattern, a compile-time error
occurs. Each of the two alternatives can itself contain nested subpatterns of any form,
including conditional subpatterns; the restriction to two alternatives applies only at the
level of the condition. The following pattern fragment is an example where the
alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpatterns, references to recursion, a
pseudo-condition called DEFINE, and assertions.
Checking for a Used Subpattern By Number
If the text between the parentheses consists of a sequence of digits, the condition is
true if a capturing subpattern of that number has previously matched. If more than one
capturing subpattern with the same number exists (see section Duplicate Subpattern
Numbers earlier), the condition is true if any of them have matched. An alternative
notation is to precede the digits with a plus or minus sign. In this case, the subpattern
number is relative rather than absolute. The most recently opened parentheses can be
referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops, it can also
make sense to refer to subsequent groups. The next parentheses to be opened can be
referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it
provokes a compile-time error.)
Consider the following pattern, which contains non-significant whitespace to make it more
readable (assume option extended) and to divide it into three parts for ease of
discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that character is present,
sets it as the first captured substring. The second part matches one or more characters
that are not parentheses. The third part is a conditional subpattern that tests whether
the first set of parentheses matched or not. If they did, that is, if subject started with
an opening parenthesis, the condition is true, and so the yes-pattern is executed and a
closing parenthesis is required. Otherwise, as no-pattern is not present, the subpattern
matches nothing. That is, this pattern matches a sequence of non-parentheses, optionally
enclosed in parentheses.
If this pattern is embedded in a larger one, a relative reference can be used:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger pattern.
Checking for a Used Subpattern By Name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by
name. For compatibility with earlier versions of PCRE, which had this facility before
Perl, the syntax (?(name)...) is also recognized.
Rewriting the previous example to use a named subpattern gives:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test is applied to all
subpatterns of the same name, and is true if any one of them has matched.
Checking for Pattern Recursion
If the condition is the string (R), and there is no subpattern with the name R, the
condition is true if a recursive call to the whole pattern or any subpattern has been
made. If digits or a name preceded by ampersand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a subpattern whose number or
name is given. This condition does not check the entire recursion stack. If the name used
in a condition of this kind is a duplicate, the test is applied to all subpatterns of the
same name, and is true if any one of them is the most recent recursion.
At "top-level", all these recursion test conditions are false. The syntax for recursive
patterns is described below.
Defining Subpatterns for Use By Reference Only
If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE,
the condition is always false. In this case, there can be only one alternative in the
subpattern. It is always skipped if control reaches this point in the pattern. The idea of
DEFINE is that it can be used to define "subroutines" that can be referenced from
elsewhere. (The use of subroutines is described below.) For example, a pattern to match an
IPv4 address, such as "192.168.23.245", can be written like this (ignore whitespace and
line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which is a another group named
"byte" is defined. This matches an individual component of an IPv4 address (a number <
256). When matching takes place, this part of the pattern is skipped, as DEFINE acts like
a false condition. The remaining pattern uses references to the named group to match the
four dot-separated components of an IPv4 address, insisting on a word boundary at each
end.
Assertion Conditions
If the condition is not in any of the above formats, it must be an assertion. This can be
a positive or negative lookahead or lookbehind assertion. Consider the following pattern,
containing non-significant whitespace, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an optional sequence of non-
letters followed by a letter. That is, it tests for the presence of at least one letter in
the subject. If a letter is found, the subject is matched against the first alternative,
otherwise it is matched against the second. This pattern matches strings in one of the two
forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
COMMENTS
There are two ways to include comments in patterns that are processed by PCRE. In both
cases, the start of the comment must not be in a character class, or in the middle of any
other sequence of related characters such as (?: or a subpattern name or number. The
characters that make up a comment play no part in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the next closing
parenthesis. Nested parentheses are not permitted. If option PCRE_EXTENDED is set, an
unescaped # character also introduces a comment, which in this case continues to
immediately after the next newline character or character sequence in the pattern. Which
characters are interpreted as newlines is controlled by the options passed to a compiling
function or by a special sequence at the start of the pattern, as described in section
Newline Conventions earlier.
Notice that the end of this type of comment is a literal newline sequence in the pattern;
escape sequences that happen to represent a newline do not count. For example, consider
the following pattern when extended is set, and the default newline convention is in
force:
abc #comment \n still comment
On encountering character #, pcre_compile() skips along, looking for a newline in the
pattern. The sequence \n is still literal at this stage, so it does not terminate the
comment. Only a character with code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for unlimited nested
parentheses. Without the use of recursion, the best that can be done is to use a pattern
that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary
nesting depth.
For some time, Perl has provided a facility that allows regular expressions to recurse
(among other things). It does this by interpolating Perl code in the expression at
runtime, and the code can refer to the expression itself. A Perl pattern using code
interpolation to solve the parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
Item (?p{...}) interpolates Perl code at runtime, and in this case refers recursively to
the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports
special syntax for recursion of the entire pattern, and for individual subpattern
recursion. After its introduction in PCRE and Python, this kind of recursion was later
introduced into Perl at release 5.10.
A special item that consists of (? followed by a number > 0 and a closing parenthesis is a
recursive subroutine call of the subpattern of the given number, if it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call, which is described in the next
section.) The special item (?R) or (?0) is a recursive call of the entire regular
expression.
This PCRE pattern solves the nested parentheses problem (assume that option extended is
set so that whitespace is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of substrings, which
can either be a sequence of non-parentheses or a recursive match of the pattern itself
(that is, a correctly parenthesized substring). Finally there is a closing parenthesis.
Notice the use of a possessive quantifier to avoid backtracking into sequences of non-
parentheses.
If this was part of a larger pattern, you would not want to recurse the entire pattern, so
instead you can use:
( \( ( [^()]++ | (?1) )* \) )
The pattern is here within parentheses so that the recursion refers to them instead of the
whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made
easier by the use of relative references. Instead of (?1) in the pattern above, you can
write (?-2) to refer to the second most recently opened parentheses preceding the
recursion. That is, a negative number counts capturing parentheses leftwards from the
point at which it is encountered.
It is also possible to refer to later opened parentheses, by writing references such as
(?+2). However, these cannot be recursive, as the reference is not inside the parentheses
that are referenced. They are always non-recursive subroutine calls, as described in the
next section.
An alternative approach is to use named parentheses instead. The Perl syntax for this is
(?&name). The earlier PCRE syntax (?P>name) is also supported. We can rewrite the above
example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the earliest one is used.
This particular example pattern that we have studied contains nested unlimited repeats,
and so the use of a possessive quantifier for matching strings of non-parentheses is
important when applying the pattern to strings that do not match. For example, when this
pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it gives "no match" quickly. However, if a possessive quantifier is not used, the match
runs for a long time, as there are so many different ways the + and * repeats can carve up
the subject, and all must be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those from the outermost
level. If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef", which is the last
value taken on at the top-level. If a capturing subpattern is not matched at the top
level, its final captured value is unset, even if it was (temporarily) set at a deeper
level during the matching process.
Do not confuse item (?R) with condition (R), which tests for recursion. Consider the
following pattern, which matches text in angle brackets, allowing for arbitrary nesting.
Only digits are allowed in nested brackets (that is, when recursing), while any characters
are permitted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
Here (?(R) is the start of a conditional subpattern, with two different alternatives for
the recursive and non-recursive cases. Item (?R) is the actual recursive call.
Differences in Recursion Processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like
Python, but unlike Perl), a recursive subpattern call is always treated as an atomic
group. That is, once it has matched some of the subject string, it is never re-entered,
even if it contains untried alternatives and there is a subsequent matching failure. This
can be illustrated by the following pattern, which means to match a palindromic string
containing an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or two identical characters
surrounding a subpalindrome. In Perl, this pattern works; in PCRE it does not work if the
pattern is longer than three characters. Consider the subject string "abcba".
At the top level, the first character is matched, but as it is not at the end of the
string, the first alternative fails, the second alternative is taken, and the recursion
kicks in. The recursive call to subpattern 1 successfully matches the next character
("b"). (Notice that the beginning and end of line tests are not part of the recursion.)
Back at the top level, the next character ("c") is compared with what subpattern 2
matched, which was "a". This fails. As the recursion is treated as an atomic group, there
are now no backtracking points, and so the entire match fails. (Perl can now re-enter the
recursion and try the second alternative.) However, if the pattern is written with the
alternatives in the other order, things are different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and continues to recurse until it
runs out of characters, at which point the recursion fails. But this time we have another
alternative to try at the higher level. That is the significant difference: in the
previous case the remaining alternative is at a deeper recursion level, which PCRE cannot
use.
To change the pattern so that it matches all palindromic strings, not only those with an
odd number of characters, it is tempting to change the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper
recursion has matched a single character, it cannot be entered again to match an empty
string. The solution is to separate the two cases, and write out the odd and even cases as
alternatives at the higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pattern must ignore all non-word
characters, which can be done as follows:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with option caseless, this pattern matches phrases such as "A man, a plan, a canal:
Panama!" and it works well in both PCRE and Perl. Notice the use of the possessive
quantifier *+ to avoid backtracking into sequences of non-word characters. Without this,
PCRE takes much longer (10 times or more) to match typical phrases, and Perl takes so long
that you think it has gone into a loop.
Note:
The palindrome-matching patterns above work only if the subject string does not start with
a palindrome that is shorter than the entire string. For example, although "abcba" is
correctly matched, if the subject is "ababa", PCRE finds palindrome "aba" at the start,
and then fails at top level, as the end of the string does not follow. Once again, it
cannot jump back into the recursion to try other alternatives, so the entire match fails.
The second way in which PCRE and Perl differ in their recursion processing is in the
handling of captured values. In Perl, when a subpattern is called recursively or as a
subpattern (see the next section), it has no access to any values that were captured
outside the recursion. In PCRE these values can be referenced. Consider the following
pattern:
^(.)(\1|a(?2))
In PCRE, it matches "bab". The first capturing parentheses match "b", then in the second
group, when the back reference \1 fails to match "b", the second alternative matches "a",
and then recurses. In the recursion, \1 does now match "b" and so the whole match
succeeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot
access the externally set value.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by name) is used
outside the parentheses to which it refers, it operates like a subroutine in a programming
language. The called subpattern can be defined before or after the reference. A numbered
reference can be absolute or relative, as in the following examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the following pattern matches "sense and sensibility"
and "response and responsibility", but not "sense and responsibility":
(sens|respons)e and \1ibility
If instead the following pattern is used, it matches "sense and responsibility" and the
other two strings:
(sens|respons)e and (?1)ibility
Another example is provided in the discussion of DEFINE earlier.
All subroutine calls, recursive or not, are always treated as atomic groups. That is, once
a subroutine has matched some of the subject string, it is never re-entered, even if it
contains untried alternatives and there is a subsequent matching failure. Any capturing
parentheses that are set during the subroutine call revert to their previous values
afterwards.
Processing options such as case-independence are fixed when a subpattern is defined, so if
it is used as a subroutine, such options cannot be changed for different calls. For
example, the following pattern matches "abcabc" but not "abcABC", as the change of
processing option does not affect the called subpattern:
(abc)(?i:(?-1))
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number
enclosed either in angle brackets or single quotes, is alternative syntax for referencing
a subpattern as a subroutine, possibly recursively. Here follows two of the examples used
above, rewritten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded by a plus or minus sign,
it is taken as a relative reference, for example:
(abc)(?i:\g<-1>)
Notice that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The
former is a back reference; the latter is a subroutine call.
BACKTRACKING CONTROL
Perl 5.10 introduced some "Special Backtracking Control Verbs", which are still described
in the Perl documentation as "experimental and subject to change or removal in a future
version of Perl". It goes on to say: "Their usage in production code should be noted to
avoid problems during upgrades." The same remarks apply to the PCRE features described in
this section.
The new verbs make use of what was previously invalid syntax: an opening parenthesis
followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some can
take either form, possibly behaving differently depending on whether a name is present. A
name is any sequence of characters that does not include a closing parenthesis. The
maximum name length is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
libraries. If the name is empty, that is, if the closing parenthesis immediately follows
the colon, the effect is as if the colon was not there. Any number of these verbs can
occur in a pattern.
The behavior of these verbs in repeated groups, assertions, and in subpatterns called as
subroutines (whether or not recursively) is described below.
Optimizations That Affect Backtracking Verbs
PCRE contains some optimizations that are used to speed up matching by running some checks
at the start of each match attempt. For example, it can know the minimum length of
matching subject, or that a particular character must be present. When one of these
optimizations bypasses the running of a match, any included backtracking verbs are not
processed. processed. You can suppress the start-of-match optimizations by setting option
no_start_optimize when calling compile/2 or run/3, or by starting the pattern with
(*NO_START_OPT).
Experiments with Perl suggest that it too has similar optimizations, sometimes leading to
anomalous results.
Verbs That Act Immediately
The following verbs act as soon as they are encountered. They must not be followed by a
name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder of the pattern.
However, when it is inside a subpattern that is called as a subroutine, only that
subpattern is ended successfully. Matching then continues at the outer level. If (*ACCEPT)
is triggered in a positive assertion, the assertion succeeds; in a negative assertion, the
assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For example,
the following matches "AB", "AAD", or "ACD". When it matches "AB", "B" is captured by the
outer parentheses.
A((?:A|B(*ACCEPT)|C)D)
The following verb causes a matching failure, forcing backtracking to occur. It is
equivalent to (?!) but easier to read.
(*FAIL) or (*F)
The Perl documentation states that it is probably useful only when combined with (?{}) or
(??{}). Those are Perl features that are not present in PCRE.
A match with the string "aaaa" always fails, but the callout is taken before each
backtrack occurs (in this example, 10 times).
Recording Which Path Was Taken
The main purpose of this verb is to track how a match was arrived at, although it also has
a secondary use in with advancing the match starting point (see (*SKIP) below).
Note:
In Erlang, there is no interface to retrieve a mark with run/2,3, so only the secondary
purpose is relevant to the Erlang programmer.
The rest of this section is therefore deliberately not adapted for reading by the Erlang
programmer, but the examples can help in understanding NAMES as they can be used by
(*SKIP).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There can be as many instances of (*MARK) as you
like in a pattern, and their names do not have to be unique.
When a match succeeds, the name of the last encountered (*MARK:NAME), (*PRUNE:NAME), or
(*THEN:NAME) on the matching path is passed back to the caller as described in section
"Extra data for pcre_exec()" in the pcreapi documentation. In the following example of
pcretest output, the /K modifier requests the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this example it indicates
which of the two alternatives matched. This is a more efficient way of obtaining this
information than putting each alternative in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is true, the name is
recorded and passed back if it is the last encountered. This does not occur for negative
assertions or failing positive assertions.
After a partial match or a failed match, the last encountered name in the entire match
process is returned, for example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Notice that in this unanchored example, the mark is retained from the match attempt that
started at letter "X" in the subject. Subsequent match attempts starting at "P" and then
with an empty string do not get as far as the (*MARK) item, nevertheless do not reset it.
Verbs That Act after Backtracking
The following verbs do nothing when they are encountered. Matching continues with what
follows, but if there is no subsequent match, causing a backtrack to the verb, a failure
is forced. That is, backtracking cannot pass to the left of the verb. However, when one of
these verbs appears inside an atomic group or an assertion that is true, its effect is
confined to that group, as once the group has been matched, there is never any
backtracking into it. In this situation, backtracking can "jump back" to the left of the
entire atomic group or assertion. (Remember also, as stated above, that this localization
also applies in subroutine calls.)
These verbs differ in exactly what kind of failure occurs when backtracking reaches them.
The behavior described below is what occurs when the verb is not in a subroutine or an
assertion. Subsequent sections cover these special cases.
The following verb, which must not be followed by a name, causes the whole match to fail
outright if there is a later matching failure that causes backtracking to reach it. Even
if the pattern is unanchored, no further attempts to find a match by advancing the
starting point take place.
(*COMMIT)
If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed,
run/2,3 is committed to find a match at the current starting point, or not at all, for
example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor,
or "I've started, so I must finish". The name of the most recently passed (*MARK) in the
path is passed back when (*COMMIT) forces a match failure.
If more than one backtracking verb exists in a pattern, a different one that follows
(*COMMIT) can be triggered first, so merely passing (*COMMIT) during a match does not
always guarantee that a match must be at this starting point.
Notice that (*COMMIT) at the start of a pattern is not the same as an anchor, unless the
PCRE start-of-match optimizations are turned off, as shown in the following example:
1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]).
{match,["abc"]}
2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]).
nomatch
For this pattern, PCRE knows that any match must start with "a", so the optimization skips
along the subject to "a" before applying the pattern to the first set of data. The match
attempt then succeeds. In the second call the no_start_optimize disables the optimization
that skips along to the first character. The pattern is now applied starting at "x", and
so the (*COMMIT) causes the match to fail without trying any other starting points.
The following verb causes the match to fail at the current starting position in the
subject if there is a later matching failure that causes backtracking to reach it:
(*PRUNE) or (*PRUNE:NAME)
If the pattern is unanchored, the normal "bumpalong" advance to the next starting
character then occurs. Backtracking can occur as usual to the left of (*PRUNE), before it
is reached, or when matching to the right of (*PRUNE), but if there is no match to the
right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an
alternative to an atomic group or possessive quantifier, but there are some uses of
(*PRUNE) that cannot be expressed in any other way. In an anchored pattern, (*PRUNE) has
the same effect as (*COMMIT).
The behavior of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). It is like
(*MARK:NAME) in that the name is remembered for passing back to the caller. However,
(*SKIP:NAME) searches only for names set with (*MARK).
Note:
The fact that (*PRUNE:NAME) remembers the name is useless to the Erlang programmer, as
names cannot be retrieved.
The following verb, when specified without a name, is like (*PRUNE), except that if the
pattern is unanchored, the "bumpalong" advance is not to the next character, but to the
position in the subject where (*SKIP) was encountered.
(*SKIP)
(*SKIP) signifies that whatever text was matched leading up to it cannot be part of a
successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails (starting at the first
character in the string), the starting point skips on to start the next attempt at "c".
Notice that a possessive quantifier does not have the same effect as this example;
although it would suppress backtracking during the first match attempt, the second attempt
would start at the second character instead of skipping on to "c".
When (*SKIP) has an associated name, its behavior is modified:
(*SKIP:NAME)
When this is triggered, the previous path through the pattern is searched for the most
recent (*MARK) that has the same name. If one is found, the "bumpalong" advance is to the
subject position that corresponds to that (*MARK) instead of to where (*SKIP) was
encountered. If no (*MARK) with a matching name is found, (*SKIP) is ignored.
Notice that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names
that are set by (*PRUNE:NAME) or (*THEN:NAME).
The following verb causes a skip to the next innermost alternative when backtracking
reaches it. That is, it cancels any further backtracking within the current alternative.
(*THEN) or (*THEN:NAME)
The verb name comes from the observation that it can be used for a pattern-based if-then-
else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items after the end of
the group if FOO succeeds). On failure, the matcher skips to the second alternative and
tries COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is
tried. If BAZ then fails, there are no more alternatives, so there is a backtrack to
whatever came before the entire group. If (*THEN) is not inside an alternation, it acts
like (*PRUNE).
The behavior of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). It is like
(*MARK:NAME) in that the name is remembered for passing back to the caller. However,
(*SKIP:NAME) searches only for names set with (*MARK).
Note:
The fact that (*THEN:NAME) remembers the name is useless to the Erlang programmer, as
names cannot be retrieved.
A subpattern that does not contain a | character is just a part of the enclosing
alternative; it is not a nested alternation with only one alternative. The effect of
(*THEN) extends beyond such a subpattern to the enclosing alternative. Consider the
following pattern, where A, B, and so on, are complex pattern fragments that do not
contain any | characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not backtrack into A;
instead it moves to the next alternative, that is, D. However, if the subpattern
containing (*THEN) is given an alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a failure in C,
matching moves to (*FAIL), which causes the whole subpattern to fail, as there are no more
alternatives to try. In this case, matching does now backtrack into A.
Notice that a conditional subpattern is not considered as having two alternatives, as only
one is ever used. That is, the | character in a conditional subpattern has a different
meaning. Ignoring whitespace, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. As .*? is ungreedy, it initially
matches zero characters. The condition (?=a) then fails, the character "b" is matched, but
"c" is not. At this point, matching does not backtrack to .*? as can perhaps be expected
from the presence of the | character. The conditional subpattern is part of the single
alternative that comprises the whole pattern, and so the match fails. (If there was a
backtrack into .*?, allowing it to match "b", the match would succeed.)
The verbs described above provide four different "strengths" of control when subsequent
matching fails:
* (*THEN) is the weakest, carrying on the match at the next alternative.
* (*PRUNE) comes next, fails the match at the current starting position, but allows an
advance to the next character (for an unanchored pattern).
* (*SKIP) is similar, except that the advance can be more than one character.
* (*COMMIT) is the strongest, causing the entire match to fail.
More than One Backtracking Verb
If more than one backtracking verb is present in a pattern, the one that is backtracked
onto first acts. For example, consider the following pattern, where A, B, and so on, are
complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail.
However, if A and B match, but C fails, the backtrack to (*THEN) causes the next
alternative (ABD) to be tried. This behavior is consistent, but is not always the same as
in Perl. It means that if two or more backtracking verbs appear in succession, the last of
them has no effect. Consider the following example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be
triggered, and its action is taken. There can never be a backtrack onto (*COMMIT).
Backtracking Verbs in Repeated Groups
PCRE differs from Perl in its handling of backtracking verbs in repeated groups. For
example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in the second
repeat of the group acts.
Backtracking Verbs in Assertions
(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed without any further
processing. In a negative assertion, (*ACCEPT) causes the assertion to fail without any
further processing.
The other backtracking verbs are not treated specially if they appear in a positive
assertion. In particular, (*THEN) skips to the next alternative in the innermost enclosing
group that has alternations, regardless if this is within the assertion.
Negative assertions are, however, different, to ensure that changing a positive assertion
into a negative assertion changes its result. Backtracking into (*COMMIT), (*SKIP), or
(*PRUNE) causes a negative assertion to be true, without considering any further
alternative branches in the assertion. Backtracking into (*THEN) causes it to skip to the
next enclosing alternative within the assertion (the normal behavior), but if the
assertion does not have such an alternative, (*THEN) behaves like (*PRUNE).
Backtracking Verbs in Subroutines
These behaviors occur regardless if the subpattern is called recursively. The treatment of
subroutines in Perl is different in some cases.
* (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an
immediate backtrack.
* (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
succeed without any further processing. Matching then continues after the subroutine
call.
* (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause the
subroutine match to fail.
* (*THEN) skips to the next alternative in the innermost enclosing group within the
subpattern that has alternatives. If there is no such group within the subpattern,
(*THEN) causes the subroutine match to fail.