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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  library's matching algorithms are currently 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.
       The  sections  of  the  PCRE  documentation which 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 additional backslash, i.e.: "\\".

DATA TYPES

       mp() = {re_pattern, term(), term(), term()}

              Opaque datatype containing a compiled regular expression. The 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 may
              change in future 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

EXPORTS

       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}

              This function compiles a regular expression with the syntax described below into an
              internal format to be used later as a parameter to the run/2,3 functions.

              Compiling the regular expression before matching is useful if the  same  expression
              is  to be used in matching against multiple subjects during the program's lifetime.
              Compiling once and executing many times is far more efficient than  compiling  each
              time one wants to match.

              When the unicode option is given, the regular expression should be given as a valid
              Unicode charlist(), otherwise as any valid iodata().

              The options have the following meanings:

                unicode:
                  The regular expression is given as  a  Unicode  charlist()  and  the  resulting
                  regular  expression  code  is  to  be  run  against  a valid Unicode charlist()
                  subject.

                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 being searched (the
                  "subject string"). This effect can also be achieved by  appropriate  constructs
                  in the pattern itself.

                caseless:
                  Letters  in  the  pattern  match  both  upper  and  lower  case  letters. It is
                  equivalent to Perl's /i option, and it 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).   The
                  dollar_endonly  option is ignored if multiline is given. There is no equivalent
                  option in Perl, and no way to set it within a pattern.

                dotall:
                  A dot in the pattern matches all  characters,  including  those  that  indicate
                  newline.  Without  it,  a  dot does not match when the current position is at a
                  newline. This option is equivalent to Perl's /s option, 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 this option's setting.

                extended:
                  Whitespace data characters in the pattern are ignored except  when  escaped  or
                  inside  a  character class. Whitespace does not include the VT character (ASCII
                  11). In addition, characters between an unescaped # outside a  character  class
                  and the next newline, inclusive, are also ignored. This is equivalent to Perl's
                  /x option, and it can be changed within a pattern by  a  (?x)  option  setting.
                  This  option makes it possible to include comments inside complicated patterns.
                  Note, however, that this applies only to data characters. Whitespace characters
                  may  never  appear within special character sequences in a pattern, for example
                  within the sequence (?( which introduces a conditional subpattern.

                firstline:
                  An unanchored pattern is required to match before or at the  first  newline  in
                  the subject string, though the matched text may continue over the newline.

                multiline:
                  By  default,  PCRE  treats the subject string as consisting of a single line of
                  characters (even if  it  actually  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 dollar_endonly is given). This is the same as Perl.

                  When multiline is given, 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's /m option, and it 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 were followed by ?: but
                  named parentheses can still be used for capturing (and they acquire numbers  in
                  the usual way). There is no equivalent of this 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. There are more details of named
                  subpatterns below

                ungreedy:
                  This option 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}:
                  Override 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 should be recognized.

                  any:
                    Any of the newline sequences above, plus 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 etc, the default).

       run(Subject, RE) -> {match, Captured} | nomatch

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 Captured = [CaptureData]
                 CaptureData = {integer(), integer()}

              The same as run(Subject,RE,[]).

       run(Subject, RE, Options) -> {match, Captured} | match | nomatch

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata() | unicode:charlist()
                 Options = [Option]
                 Option = anchored
                        | global
                        | notbol
                        | noteol
                        | notempty
                        | {offset, 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 | first | none | ValueList
                 ValueList = [ValueID]
                 ValueID = integer() | string() | atom()
                 CompileOpt = compile_option()
                   See compile/2 above.
                 Captured = [CaptureData] | [[CaptureData]]
                 CaptureData = {integer(), integer()}
                             | ListConversionData
                             | binary()
                 ListConversionData = string()
                                    | {error, string(), binary()}
                                    | {incomplete, string(), binary()}

              Executes a regexp matching,  returning  match/{match,  Captured}  or  nomatch.  The
              regular   expression  can  be  given  either  as  iodata()  in  which  case  it  is
              automatically compiled (as by re:compile/2) and executed, or as a pre-compiled mp()
              in which case it is executed against the subject directly.

              When compilation is involved, the exception badarg is thrown if a compilation error
              occurs. Call re: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 options anchored, global, notbol, noteol, notempty, {offset, integer()  >=  0},
              {newline,  NLSpec}  and  {capture, ValueSpec}/{capture, ValueSpec, Type}. Otherwise
              all options valid for the  re:compile/2  function  are  allowed  as  well.  Options
              allowed  both  for  compilation  and  execution  of  a  match,  namely anchored and
              {newline, NLSpec}, will affect  both  the  compilation  and  execution  if  present
              together with a non pre-compiled regular expression.

              If  the  regular  expression  was  previously compiled with the option unicode, the
              Subject should be provided as a valid Unicode charlist(),  otherwise  any  iodata()
              will  do.  If  compilation  is  involved  and the option unicode is given, both the
              Subject and the regular expression should be given as valid Unicode charlists().

              The {capture, ValueSpec}/{capture, ValueSpec, Type} defines what to return from the
              function  upon  successful  matching.  The  capture  tuple may 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 capture option makes the  function  quite  flexible
              and powerful. The different options are described in detail below.

              If  the  capture  options describe that no substring capturing at all is to be done
              ({capture, none}), the function will return the single atom match  upon  successful
              matching,  otherwise  the tuple {match, ValueList} is returned. Disabling capturing
              can be done either by specifying none or an empty list as ValueSpec.

              The options relevant for execution are:

                anchored:
                  Limits re: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  (the  g  flag in Perl). Each match is
                  returned as a separate list() containing the specific  match  as  well  as  any
                  matching  subexpressions  (or as specified by the capture option). The Captured
                  part of the return value will hence be a list() of list()s when this option  is
                  given.

                  The interaction of the global option with a regular expression which matches an
                  empty string surprises some users. When the global option  is  given,  re:run/3
                  handles empty matches in the same way as Perl: a zero-length match at any point
                  will be retried with the options [anchored, notempty] as well. If  that  search
                  gives a result of length > 0, the result is included. For example:

                    re:run("cat","(|at)",[global]).

                  The following matching will be performed:

                  At offset 0:
                    The  regexp (|at) will first match at the initial position of the string cat,
                    giving the  result  set  [{0,0},{0,0}]  (the  second  {0,0}  is  due  to  the
                    subexpression marked by the parentheses). As the length of the match is 0, we
                    don't advance to the next position yet.

                  At offset 0 with [anchored, notempty]:
                     The search is retried with the options  [anchored,  notempty]  at  the  same
                    position, which does not give any interesting result of longer length, so the
                    search position is now advanced to the next character (a).

                  At offset 1:
                    This time, the search results in [{1,0},{1,0}], so this search will  also  be
                    repeated with the extra options.

                  At offset 1 with [anchored, notempty]:
                    Now  the  ab  alternative  is found and the result will be [{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 now once again matches the empty string, giving [{3,0},{3,0}].

                  At offset 1 with [anchored, notempty]:
                    This  will  give  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  given.
                  If  there  are  alternatives  in  the  pattern,  they  are  tried.  If  all the
                  alternatives match the empty string, the entire match fails.  For  example,  if
                  the pattern

                    a?b?

                  is  applied  to a string not beginning with "a" or "b", it would normally match
                  the empty string at the start of the subject. With the  notempty  option,  this
                  match  is  not  valid,  so  re:run/3  searches  further  into  the  string  for
                  occurrences of "a" or "b".

                  Perl has no direct equivalent of notempty, but it does make a special case of a
                  pattern  match  of the empty string within its split() function, and when using
                  the /g modifier. It is possible to emulate Perl's  behavior  after  matching  a
                  null  string  by  first trying the match again at the same offset with notempty
                  and anchored, and then, if that fails, by advancing the  starting  offset  (see
                  below) and trying an ordinary match again.

                notbol:
                  This option specifies that the first character of the subject string is not the
                  beginning of a line, so the circumflex metacharacter should  not  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:
                  This  option  specifies  that the end of the subject string is not the end of a
                  line, so the dollar metacharacter should not 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.

                {offset, integer() >= 0}:
                  Start matching at the offset (position) given in the subject string. The offset
                  is zero-based, so that the default is {offset,0} (all of the subject string).

                {newline, NLSpec}:
                  Override 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 should be recognized.

                  any:
                    Any of the newline sequences above, plus 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 compilation option)

                bsr_unicode:
                  Specifies  specifically  that \R is to match all the Unicode newline characters
                  (including crlf etc, the default).(overrides compilation option)

                {capture, ValueSpec}/{capture, ValueSpec, Type}:
                  Specifies which captured  substrings  are  returned  and  in  what  format.  By
                  default, re:run/3 captures all of the matching part of the substring as well as
                  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, given as {Offset,Length} pairs (the index Type of capturing).

                  As an example of the default behavior, the following call:

                    re:run("ABCabcdABC","abcd",[]).

                  returns, as first and only captured string the matching  part  of  the  subject
                  ("abcd"  in  the  middle)  as a index pair {3,4}, where character positions are
                  zero based, just as in offsets. The return value of the call above  would  then
                  be:

                    {match,[{3,4}]}

                  Another  (and quite common) case is where the regular expression matches all of
                  the subject, as in:

                    re:run("ABCabcdABC",".*abcd.*",[]).

                  where the return value correspondingly  will  point  out  all  of  the  string,
                  beginning at index 0 and being 10 characters long:

                    {match,[{0,10}]}

                  If the regular expression contains capturing subpatterns, like in the following
                  case:

                    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 giving the first return value in the  list
                  and  the  rest of the subpatterns being 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. The ValueSpec  can
                    either  be  an  atom  describing a predefined set of return values, or a list
                    containing either the indexes or the names of specific subpatterns to return.

                    The predefined sets of subpatterns are:

                    all:
                      All captured subpatterns including the complete matching  string.  This  is
                      the default.

                    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,  i.e.  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're interested in is in an explicitly captured
                      subpattern. If the return type is list or binary, not returning subpatterns
                      you're not interested in is a good way to optimize.

                    none:
                      Do  not  return matching subpatterns at all, yielding 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 forth. 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 the string "ABCabcdABC", capturing only the "abcd" part (the
                    first explicit subpattern):

                      re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).

                    The call will yield the following result:

                      {match,[{3,4}]}

                    as the first explicitly captured subpattern is "(abcd)", matching  "abcd"  in
                    the subject, at (zero-based) position 3, of length 4.

                    Now  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, since the subpattern is named, we can
                    also specify its name in the value list:

                      re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).

                    which would yield the same result as the earlier examples, namely:

                      {match,[{3,4}]}

                    The values list might 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  having  no
                    corresponding  subpattern  in the regexp, 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. The Type can be one of the following:

                    index:
                      Return 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  iolist_to_binary/1  or unicode:characters_to_binary/2
                      prior to matching). Note that the unicode option results  in  byte-oriented
                      indexes  in  a  (possibly virtual) UTF-8 encoded binary. A byte index tuple
                      {0,2} might therefore represent one or two characters when  unicode  is  in
                      effect.  This  might  seem  counter-intuitive, but has been deemed the most
                      effective and useful way to way to do it. To  return  lists  instead  might
                      result in simpler code if that is desired. This return type is the default.

                    list:
                      Return  matching  substrings  as lists of characters (Erlang string()s). It
                      the unicode option 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  may  result  in  the  same types of tuples that
                      unicode:characters_to_list/2 can return, namely three-tuples with  the  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:
                      Return  matching  substrings  as  binaries.  If the unicode option is used,
                      these binaries are in UTF-8. If the  \C  sequence  is  used  together  with
                      unicode the binaries may 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 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 won't match, as "abdd" is not present in the string,
                  but the complete pattern matches (due to the alternative a(..d). The subpattern
                  at index 2 is therefore unassigned and the default return value will be:

                    {match,[{0,10},{3,4},{-1,0},{4,3}]}

                  Setting the capture Type to binary would give the following:

                    {match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}

                  where  the  empty  binary  (<<>>)  represents the unassigned subpattern. In the
                  binary case, some information about the matching is therefore  lost,  the  <<>>
                  might just as well 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  the  option global is given, the capture specification affects each match
                  separately, so that:

                    re:run("cacb","c(a|b)",[global,{capture,[1],list}]).

                  gives the result:

                    {match,[["a"],["b"]]}

              The options solely affecting the compilation step are described in the re:compile/2
              function.

       replace(Subject, RE, Replacement) -> iodata() | unicode:charlist()

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 Replacement = iodata() | unicode:charlist()

              The 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
                        | {offset, integer() >= 0}
                        | {newline, NLSpec}
                        | bsr_anycrlf
                        | 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 re:run/3, except that the capture
              option 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} and if a binary is preferred, specify {return, binary}.

              As  in the re:run/3 function, an mp() compiled with the unicode option requires the
              Subject to be a Unicode charlist(). If  compilation  is  done  implicitly  and  the
              unicode  compilation  option is given to this function, both the regular expression
              and the Subject should be given 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 will be inserted
              in  the  result.  If  no subexpression with that number is generated by the regular
              expression, nothing is inserted.

              To insert an & or \ in the result, precede it with a \. Note  that  Erlang  already
              gives a special meaning to \ in literal strings, so a single \ has to 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 re:run/3, compilation errors raise the badarg exception,  re:compile/2  can
              be used to get more information about the error.

       split(Subject, RE) -> SplitList

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 SplitList = [iodata() | unicode:charlist()]

              The 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
                        | {offset, integer() >= 0}
                        | {newline, nl_spec()}
                        | 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 above.
                 SplitList = [RetData] | [GroupedRetData]
                 GroupedRetData = [RetData]
                 RetData = iodata() | unicode:charlist() | binary() | list()

              This  function  splits  the  input  into  parts  by finding tokens according to the
              regular expression supplied.

              The splitting is done basically by running a global regexp match and  dividing  the
              initial  string wherever a match occurs. The matching part of the string is removed
              from the output.

              As in the re:run/3 function, an mp() compiled with the unicode option requires  the
              Subject  to  be  a  Unicode  charlist().  If compilation is done implicitly and the
              unicode compilation option is given to this function, both the  regular  expression
              and the Subject should be given as valid Unicode charlist()s.

              The  result  is  given  as a list of "strings", the preferred datatype given in the
              return option (default iodata).

              If subexpressions are given in the regular expression, the matching  subexpressions
              are returned in the resulting list as well. An example:

                  re:split("Erlang","[ln]",[{return,list}]).

              will yield the result:

                  ["Er","a","g"]

              while

                  re:split("Erlang","([ln])",[{return,list}]).

              will yield

                  ["Er","l","a","n","g"]

              The  text  matching  the subexpression (marked by the parentheses in the regexp) is
              inserted in the result  list  where  it  was  found.  In  effect  this  means  that
              concatenating   the  result  of  a  split  where  the  whole  regexp  is  a  single
              subexpression (as in the example above) will always result in the original string.

              As there is no matching subexpression for the last part in the example  (the  "g"),
              there  is  nothing  inserted after that. To make the group of strings and the parts
              matching the subexpressions more obvious, one might use  the  group  option,  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 matched first 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. Since the
              subexpression is bound to the substring "n" in this case, the "n" is inserted  into
              this  group.  The last group consists of the rest of the 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}]).

              will return:

                  ["Er","an",[]]

              since  the  matching of the "g" in the end of the string leaves an empty rest which
              is also returned. This behaviour differs from the default behaviour  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]).

              The result will be:

                  ["Er","an"]

              The "trim" option in effect says; "give me as many parts  as  possible  except  the
              empty  ones", which might be useful in some circumstances. You can also specify how
              many parts you want, by specifying {parts,N}:

                  re:split("Erlang","[lg]",[{return,list},{parts,2}]).

              This will give:

                  ["Er","ang"]

              Note that the last part is "ang", not "an", as we only specified splitting 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}]).

              will give 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 the option trim. If
              subexpressions are captured, empty  subexpression  matches  at  the  end  are  also
              stripped from the result if trim or {parts,0} is specified.

              If  you  are familiar with Perl, the trim behaviour corresponds exactly to the Perl
              default, the {parts,N} where N is a positive integer  corresponds  exactly  to  the
              Perl  behaviour with a positive numerical third parameter and the default behaviour
              of re:split/3 corresponds to that when the Perl routine is given a negative integer
              as the third parameter.

              Summary of options not previously described for the re:run/3 function:

                {return,ReturnType}:
                  Specifies  how  the  parts  of  the original string are presented in the result
                  list. The possible types are:

                  iodata:
                    The variant of iodata() that gives the least copying of data with the current
                    implementation (often a binary, but don't 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 regexp.

                  The return value from the function will in this case be 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 should be a positive integer for a specific maximum on the
                  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 behaviour of
                  the split built in function in Perl.

PERL LIKE REGULAR EXPRESSIONS SYNTAX

       The following sections contain reference material for the regular expressions used by this
       module.  The regular expression reference is based on the PCRE documentation, with changes
       in cases where the re module behaves differently to the PCRE library.

PCRE REGULAR EXPRESSION DETAILS

       The syntax and semantics of the  regular  expressions  that  are  supported  by  PCRE  are
       described   in  detail  below.  Perl's  regular  expressions  are  described  in  its  own
       documentation, and regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples.  Jeffrey  Friedl's  "Mastering  Regular  Expressions",
       published by O'Reilly, covers regular expressions in great  detail.  This  description  of
       PCRE's regular expressions is intended as reference material.

       The reference material is divided into the following sections:

         * Newline conventions

         * Characters and metacharacters

         * Backslash

         * Circumflex and dollar

         * Full stop (period, dot)

         * Matching a single byte

         * 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

         * Backtracking control

NEWLINE CONVENTIONS

       PCRE  supports  five different conventions for indicating line breaks in strings: a single
       CR (carriage return) character,  a  single  LF  (linefeed)  character,  the  two-character
       sequence CRLF , any of the three preceding, or any Unicode newline sequence.

       It  is also possible to specify a newline convention by starting a pattern string with one
       of the following five sequences:

         (*CR):
           carriage return

         (*LF):
           linefeed

         (*CRLF):
           carriage return, followed by linefeed

         (*ANYCRLF):
           any of the three above

         (*ANY):
           all Unicode newline sequences

       These override the default and  the  options  given  to  re:compile/2.  For  example,  the
       pattern:

       (*CR)a.b

       changes  the  convention  to  CR.  That  pattern  matches "a\nb" because LF is no longer a
       newline. Note 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.

       The newline convention 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 the section entitled "Newline  sequences"  below.  A
       change of \R setting can be combined with a change of newline convention.

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 pattern

       The quick brown fox

       matches  a portion of a subject string that is identical to itself. When caseless matching
       is specified (the caseless option), 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.

       There are two different sets of metacharacters: 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 several 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 quantifier

         +:
           1 or more quantifier, also "possessive quantifier"

         {:
           start min/max quantifier

       Part of a pattern that is in square brackets is called a "character class". In a character
       class the only metacharacters are:

         \:
           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 of the metacharacters.

BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by a non-alphanumeric
       character, it takes away any  special  meaning  that  character  may  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  whether  or  not  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, you write \\.

       If  a  pattern is compiled with the extended option, 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.

       If you want to remove the special meaning from a sequence of characters, you can do so  by
       putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
       literals in  \Q...\E  sequences  in  PCRE,  whereas  in  Perl,  $  and  @  cause  variable
       interpolation. Note 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.

       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, but when a pattern is being prepared
       by text editing, it is usually 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 character

         \e :
           escape (hex 1B)

         \f:
           formfeed (hex 0C)

         \n:
           linefeed (hex 0A)

         \r:
           carriage return (hex 0D)

         \t :
           tab (hex 09)

         \ddd:
           character with octal code ddd, or backreference

         \xhh :
           character with hex code hh

         \x{hhh..}:
           character with hex code hhh..

       The  precise  effect of \cx is as follows: if x is a lower case letter, it is converted to
       upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A, but
       \c{ becomes hex 3B, while \c; becomes hex 7B.

       After  \x,  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 }, but the value of the
       character code must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode.
       That is, the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger  than  the
       largest Unicode code point, which is 10FFFF.

       If  characters  other  than hexadecimal digits appear between \x{ and }, or if there is no
       terminating }, this form of escape is not recognized. Instead,  the  initial  \x  will  be
       interpreted  as  a  basic hexadecimal escape, with no following digits, giving a character
       whose value is zero.

       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}.

       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\07 specifies two binary zeros
       followed by a BEL character (code value 7). Make sure you  supply  two  digits  after  the
       initial zero if the pattern character that follows is itself an octal digit.

       The  handling  of  a  backslash followed by a digit other than 0 is complicated. Outside a
       character class, PCRE reads it and any following digits as a decimal number. If the number
       is  less  than  10,  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 given later, following the discussion of parenthesized
       subpatterns.

       Inside a character class, or if the decimal number is greater than 9 and  there  have  not
       been that many capturing subpatterns, PCRE re-reads up to three octal digits following the
       backslash, and uses them to generate a data character. Any  subsequent  digits  stand  for
       themselves.  The  value  of a character specified in octal must be less than \400. In non-
       UTF-8 mode, the value of a character specified in octal must be less than \400.  In  UTF-8
       mode, values up to \777 are permitted. For example:

         \040:
           is another way of writing a space

         \40:
           is the same, provided there are fewer than 40 previous capturing subpatterns

         \7:
           is always a back reference

         \11:
            might be a back reference, or another way of writing a tab

         \011:
           is always a tab

         \0113:
           is a tab followed by the character "3"

         \113:
           might be a back reference, otherwise the character with octal code 113

         \377:
           might be a back reference, otherwise the byte consisting entirely of 1 bits

         \81:
           is  either  a  back reference, or a binary zero followed by the two characters "8" and
           "1"

       Note that octal values of 100 or greater must not be introduced by a leading zero, because
       no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both inside and outside
       character classes. In addition, inside a character class, the sequence \b  is  interpreted
       as  the  backspace  character (hex 08), and the sequences \R and \X are interpreted as the
       characters "R" and "X", respectively. Outside a  character  class,  these  sequences  have
       different meanings (see below).

       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.

       Generic character types

       Another use of backslash is for specifying generic  character  types.  The  following  are
       always recognized:

         \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

       Each  pair of escape sequences partitions the complete set of characters into two disjoint
       sets. Any given character matches one, and only one, of each pair.

       These character type 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 of them fail, since there is no character to match.

       For compatibility with Perl, \s does not match the VT character (code 11). This  makes  it
       different from the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR
       (13), and space (32). If "use locale;" is included in a Perl script, \s may match  the  VT
       character. In PCRE, it never does.

       In  UTF-8  mode,  characters  with  values greater than 128 never match \d, \s, or \w, and
       always match \D, \S, and \W. This is true even when Unicode character property support  is
       available.  These  sequences  retain their original meanings from before UTF-8 support was
       available, mainly for efficiency reasons.

       The sequences \h, \H, \v, and \V  are  Perl  5.10  features.  In  contrast  to  the  other
       sequences,  these  do  match  certain high-valued codepoints in UTF-8 mode. The horizontal
       space characters are:

         U+0009:
           Horizontal tab

         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 vertical space characters are:

         U+000A:
           Linefeed

         U+000B:
           Vertical tab

         U+000C:
           Formfeed

         U+000D:
           Carriage return

         U+0085:
           Next line

         U+2028:
           Line separator

         U+2029:
           Paragraph separator

       A "word" character is an underscore or any character less than 256 that  is  a  letter  or
       digit.  The  definition of letters and digits is controlled by PCRE's low-valued character
       tables, which are always ISO-8859-1.

       Newline sequences

       Outside a character class, by default, the escape sequence \R matches any Unicode  newline
       sequence.  This  is  a  Perl  5.10  feature.  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 of which are given below.

       This particular group matches either the two-character sequence CR followed by LF, or  one
       of  the  single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (formfeed,
       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  UTF-8 mode, two additional characters whose codepoints are greater than 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.

       It  is  possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set
       of Unicode line endings) by setting the option bsr_anycrlf either at compile time or  when
       the  pattern  is matched. (BSR is an abbreviation for "backslash R".) This can be made the
       default when PCRE is built; if this is the case, the other behaviour can be requested  via
       the  bsr_unicode  option.  It  is  also  possible  to specify these settings 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  given  to  re:compile/2,  but  they  can  be
       overridden  by  options given to re:run/3. Note 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)

       Inside a character class, \R matches the letter "R".

       Unicode character properties

       When PCRE is built with  Unicode  character  property  support,  three  additional  escape
       sequences  that match characters with specific properties are available. When not in UTF-8
       mode, these sequences are of course limited to testing  characters  whose  codepoints  are
       less than 256, but they do work in this mode. The extra escape sequences are:

       \p{xx}  a  character with the xx property \P{xx} a character without the xx property \X an
       extended Unicode sequence

       The property names represented by xx above are limited to the Unicode  script  names,  the
       general  category  properties, and "Any", which matches any character (including newline).
       Other properties such as "InMusicalSymbols" are not currently supported by PCRE. Note that
       \P{Any} does not match any characters, so 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
       current list of scripts is:

         * Arabic

         * Armenian

         * Balinese

         * Bengali

         * Bopomofo

         * Braille

         * Buginese

         * Buhid

         * Canadian_Aboriginal

         * Cherokee

         * Common

         * Coptic

         * Cuneiform

         * Cypriot

         * Cyrillic

         * Deseret

         * Devanagari

         * Ethiopic

         * Georgian

         * Glagolitic

         * Gothic

         * Greek

         * Gujarati

         * Gurmukhi

         * Han

         * Hangul

         * Hanunoo

         * Hebrew

         * Hiragana

         * Inherited

         * Kannada

         * Katakana

         * Kharoshthi

         * Khmer

         * Lao

         * Latin

         * Limbu

         * Linear_B

         * Malayalam

         * Mongolian

         * Myanmar

         * New_Tai_Lue

         * Nko

         * Ogham

         * Old_Italic

         * Old_Persian

         * Oriya

         * Osmanya

         * Phags_Pa

         * Phoenician

         * Runic

         * Shavian

         * Sinhala

         * Syloti_Nagri

         * Syriac

         * Tagalog

         * Tagbanwa

         * Tai_Le

         * Tamil

         * Telugu

         * Thaana

         * Thai

         * Tibetan

         * Tifinagh

         * Ugaritic

         * Yi

       Each  character  has  exactly  one  general  category  property, specified by a two-letter
       abbreviation. 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;  these  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:
           Lower case letter

         Lm:
           Modifier letter

         Lo:
           Other letter

         Lt:
           Title case letter

         Lu:
           Upper case 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, in other words, 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 not valid in UTF-8 strings (see RFC 3629) and so cannot be tested by PCRE,
       unless UTF-8 validity checking has been turned off (see the discussion of no_utf8_check in
       the pcreapi page).

       The  long  synonyms  for  these properties that Perl supports (such as \p{Letter}) are not
       supported by PCRE, nor is it permitted to prefix any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) property. Instead,  this
       property is 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 upper case letters.

       The \X escape matches any number of Unicode  characters  that  form  an  extended  Unicode
       sequence. \X is equivalent to

       (?>\PM\pM*)

       That  is,  it  matches  a  character without the "mark" property, followed by zero or more
       characters with the "mark" property, and treats the  sequence  as  an  atomic  group  (see
       below).  Characters  with  the  "mark"  property  are  typically  accents  that affect the
       preceding character. None of them have codepoints less than 256, so in non-UTF-8  mode  \X
       matches any one character.

       Matching  characters  by  Unicode  property  is  not  fast,  because  PCRE has to search a
       structure that contains data for  over  fifteen  thousand  characters.  That  is  why  the
       traditional escape sequences such as \d and \w do not use Unicode properties in PCRE.

       Resetting the match start

       The  escape  sequence  \K,  which  is  a  Perl 5.10 feature, causes any previously matched
       characters not to be included in the final matched sequence. For example, the pattern:

       foo\Kbar

       matches "foobar", but reports that it has matched "bar". 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 pattern

       (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Simple assertions

       The final use of backslash is for certain simple  assertions.  An  assertion  specifies  a
       condition  that  has  to  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 backslashed assertions are:

         \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 also matches 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

       These assertions may not appear in character classes (but note that  \b  has  a  different
       meaning, namely the backspace character, inside a character class).

       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 (i.e. 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.

       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 the notbol or noteol options, which affect only the
       behaviour of the  circumflex  and  dollar  metacharacters.  However,  if  the  startoffset
       argument  of  re: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 as well as at the very end,
       whereas \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 the startoffset argument of re:run/3. It differs from \A when
       the value of startoffset is non-zero. By calling re:run/3 multiple times with  appropriate
       arguments,  you can mimic Perl's /g option, and it is in this kind of implementation where
       \G can be useful.

       Note, however, that PCRE's interpretation of \G, as the start of  the  current  match,  is
       subtly  different from Perl's, which defines it as the end of the previous match. In Perl,
       these can be different when the previously matched string was  empty.  Because  PCRE  does
       just one match at a time, it cannot reproduce this behaviour.

       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

       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 the startoffset argument of re:run/3 is non-zero, circumflex can never match if
       the multiline option is unset. Inside  a  character  class,  circumflex  has  an  entirely
       different meaning (see below).

       Circumflex  need not be the first character of the pattern if a number of alternatives are
       involved, but it should 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.)

       A 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). Dollar need not be the  last  character  of  the  pattern  if  a  number  of
       alternatives  are  involved,  but  it  should  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 the dollar_endonly option at compile time. This does not affect the \Z
       assertion.

       The meanings of the circumflex and dollar characters are changed if the  multiline  option
       is set. When this is the case, a circumflex matches immediately after internal newlines as
       well as 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, as well as 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.  Consequently,  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 the  startoffset  argument  of
       re:run/3 is non-zero. The dollar_endonly option is ignored if multiline is set.

       Note  that  the  sequences  \A,  \Z,  and \z can be used to match the start and end of the
       subject in both modes, and if all branches of  a  pattern  start  with  \A  it  is  always
       anchored, whether or not multiline is set.

FULL STOP (PERIOD, DOT)

       Outside  a  character class, a dot in the pattern matches any one character in the subject
       string except (by default) a character that signifies the end of a line.  In  UTF-8  mode,
       the matched character may be more than one byte long.

       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, but otherwise it matches all characters (including isolated CRs and LFs).
       When any Unicode line endings are being recognized, dot does not match CR or LF or any  of
       the other line ending characters.

       The  behaviour of dot with regard to newlines can be changed. If the dotall option is set,
       a dot matches any one 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 being that they both involve newlines. Dot has no special meaning  in  a
       character class.

MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte, both in and out of
       UTF-8 mode. Unlike a dot, it always matches any line-ending  characters.  The  feature  is
       provided  in  Perl  in order to match individual bytes in UTF-8 mode. Because it breaks up
       UTF-8 characters into individual bytes, what remains in the  string  may  be  a  malformed
       UTF-8 string. For this reason, the \C escape sequence is best avoided.

       PCRE  does  not  allow \C to appear in lookbehind assertions (described below), because in
       UTF-8 mode this would make it impossible to calculate the length of the lookbehind.

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. If a closing square bracket
       is required as a member of the class, it should 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 UTF-8 mode, the character
       may occupy more than one byte. 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  actually  required  as  a  member of the class, ensure it is not the first
       character, or escape it with a backslash.

       For example, the character class [aeiou] matches any  lower  case  vowel,  while  [^aeiou]
       matches  any  character  that  is not a lower case vowel. Note 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 greater than 255 can be included in a class as a
       literal string of bytes, or by using the \x{ escaping mechanism.

       When caseless matching is set, any letters in a class represent both their upper case  and
       lower  case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a
       caseless [^aeiou] does not match "A", whereas a caseful version would. In UTF-8 mode, PCRE
       always  understands  the concept of case for characters whose values are less than 128, so
       caseless matching is always possible. For characters with higher values,  the  concept  of
       case is supported if PCRE is compiled with Unicode property support, but not otherwise. If
       you want to use caseless matching for characters 128 and above, you must ensure that  PCRE
       is compiled with Unicode property support as well as with UTF-8 support.

       Characters  that  might  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  the  dotall  and multiline options 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.

       It is not possible to have the literal character "]" as 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  the  "]"  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.

       Ranges  operate  in  the collating sequence of character values. They can also be used for
       characters specified numerically, for example  [\000-\037].  In  UTF-8  mode,  ranges  can
       include characters whose values are greater than 255, for example [\x{100}-\x{2ff}].

       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  ,  and  in non-UTF-8 mode, if character tables for a French locale are in use,
       [\xc8-\xcb] matches accented E characters in both cases. In UTF-8 mode, PCRE supports  the
       concept  of case for characters with values greater than 128 only when it is compiled with
       Unicode property support.

       The character types \d, \D, \p, \P, \s, \S, \w, and \W may  also  appear  in  a  character
       class,  and  add  the  characters  that  they  match to the class. For example, [\dABCDEF]
       matches any hexadecimal digit. A circumflex can conveniently be used with the  upper  case
       character  types  to  specify  a more restricted set of characters than the matching lower
       case type. For example, the class [^\W_] matches any letter or digit, but not underscore.

       The only metacharacters that are recognized in character  classes  are  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
       -  see  the  next  section), and the 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,

       [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class names are

         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:
           lower case letters

         print:
           printing characters, including space

         punct:
           printing characters, excluding letters and digits

         space:
           whitespace (not quite the same as \s)

         upper:
           upper case letters

         word:
           "word" characters (same as \w)

         xdigit:
           hexadecimal digits

       The  "space"  characters  are  HT (9), LF (10), VT (11), FF (12), CR (13), and space (32).
       Notice that this list includes the VT character (code 11). This makes "space" different to
       \s, which does not include VT (for Perl compatibility).

       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,

       [12[:^digit:]]

       matches "1", "2", or any non-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.

       In  UTF-8  mode,  characters  with  values  greater than 128 do not match any of the POSIX
       character classes.

VERTICAL BAR

       Vertical bar characters are used  to  separate  alternative  patterns.  For  example,  the
       pattern

       gilbert|sullivan

       matches  either  "gilbert"  or  "sullivan".  Any number of alternatives may 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 one that succeeds is used. If
       the alternatives are within a subpattern (defined below), "succeeds"  means  matching  the
       rest of the main pattern as well as the alternative in the subpattern.

INTERNAL OPTION SETTING

       The  settings  of  the  caseless, multiline, dotall, and extended options (which are Perl-
       compatible) can be changed from within the pattern by a sequence of  Perl  option  letters
       enclosed between "(?" and ")". The option letters are

         i:
           for caseless

         m:
           for multiline

         s:
           for dotall

         x:
           for extended

       For  example,  (?im) sets caseless, multiline matching. It is also possible to unset these
       options by preceding the letter with a hyphen, and 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  an  option  change occurs at top level (that is, not inside subpattern parentheses),
       the change applies to the remainder of the pattern that follows. If the change  is  placed
       right at the start of a pattern, PCRE extracts it into the global options

       An  option change within a subpattern (see below for a description of subpatterns) affects
       only that part of the current pattern that follows it, so

       (a(?i)b)c

       matches abc and aBc and no other strings (assuming caseless is not used). 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",  even  though when matching "C" the first branch is
       abandoned before the option setting. This is because the effects of option settings happen
       at compile time. There would be some very weird behaviour otherwise.

       Note:  There  are  other PCRE-specific options that can be set by the application when the
       compile or match functions are called. In some  cases  the  pattern  can  contain  special
       leading  sequences  to  override  what the application has set or what has been defaulted.
       Details are given in the section entitled "Newline sequences" above.

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 pattern

       cat(aract|erpillar|)

       matches  one of the words "cat", "cataract", or "caterpillar". Without the parentheses, it
       would match "cataract", "erpillar" or an empty string.

       2. It sets up the subpattern as a capturing subpattern. This means that, when the complete
       pattern  matches, that portion of the subject string that matched the subpattern is passed
       back to the caller via the return value of re: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 pattern

       the ((red|white) (king|queen))

       the  captured  substrings are "red king", "red", and "king", and are numbered 1, 2, and 3,
       respectively.

       The fact that plain parentheses fulfil two functions is  not  always  helpful.  There  are
       often  times when 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 pattern

       the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and  are  numbered  1  and  2.  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 may appear between the "?" and the ":". Thus  the
       two patterns

         * (?i:saturday|sunday)

         * (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because 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
       "SUNDAY" as well as "Saturday".

DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby 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 this pattern:

       (?|(Sat)ur|(Sun))day

       Because 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  part,  but  not  all,  of  one  of  a number of 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 buffers that follow the subpattern start after the highest
       number used in any branch. The following example is taken from the Perl documentation. The
       numbers underneath show in which buffer the captured content will be 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  backreference  or  a recursive call to a numbered subpattern always refers to the first
       one in the pattern with the given number.

       An alternative approach to 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 very hard to keep
       track of the numbers in complicated regular expressions. Furthermore, if an expression  is
       modified,  the  numbers may 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.

       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 backreferences, recursion, and conditions, can  be  made  by
       name as well as by number.

       Names  consist  of  up  to  32  alphanumeric  characters  and underscores. Named capturing
       parentheses are still allocated numbers as well as names, exactly as if the names were not
       present. The capture specification to re: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  it  is  possible  to  relax  this
       constraint by setting the dupnames option at compile time. This can be useful for patterns
       where only one instance of the named parentheses can match. Suppose 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. This 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.)

       In  case  of  capturing  named  subpatterns  which are not unique, the first occurrence is
       returned from re:exec/3, if the name is specified int  the  values  part  of  the  capture
       statement.

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 (in UTF-8 mode with Unicode properties)

         * the \R escape sequence

         * an escape such as \d that matches a single character

         * a character class

         * a back reference (see next section)

         * a parenthesized subpattern (unless it is an assertion)

       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 less than 65536, and the first must be less than or equal to the second.
       For example:

       z{2,4}

       matches "zz", "zzz", or "zzzz". 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

       [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

       \d{8}

       matches  exactly  8  digits.  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 UTF-8 mode, quantifiers apply to UTF-8 characters  rather  than  to  individual  bytes.
       Thus,  for  example, \x{100}{2} matches two UTF-8 characters, each of which is represented
       by a two-byte sequence. Similarly, when  Unicode  property  support  is  available,  \X{3}
       matches  three  Unicode  extended  sequences, each of which may be several bytes long (and
       they may 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.

       For convenience, the three most common quantifiers have single-character abbreviations:

         *:
           is equivalent to {0,}

         +:
           is equivalent to {1,}

         ?:
           is equivalent to {0,1}

       It  is  possible  to  construct infinite loops 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, because there are cases where this can be useful, such patterns are now accepted,
       but if any repetition of the subpattern does in fact match  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 rest of the pattern  to  fail.
       The  classic  example  of  where  this  gives problems is in trying to match comments in C
       programs. These appear between /* and */ and  within  the  comment,  individual  *  and  /
       characters may appear. An attempt to match C comments by applying the pattern

       /\*.*\*/

       to the string

       /* first comment */ not comment /* second comment */

       fails, because 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 pattern

       /\*.*?\*/

       does the right thing with the C comments. The meaning of the various  quantifiers  is  not
       otherwise  changed,  just  the  preferred  number  of  matches. Do not confuse this use of
       question mark with its use as a quantifier in its own right. Because 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 rest
       of the pattern matches.

       If the ungreedy option 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. In other words, it inverts the default behaviour.

       When a parenthesized subpattern is quantified with a minimum repeat count that is  greater
       than  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 the dotall option (equivalent to  Perl's  /s)  is
       set,  thus allowing the dot to match newlines, the pattern is implicitly anchored, because
       whatever follows will be 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 though it were preceded by \A.

       In cases where it is known that the subject string  contains  no  newlines,  it  is  worth
       setting  dotall in order to obtain this optimization, or alternatively using ^ to indicate
       anchoring explicitly.

       However, there is one situation where the optimization cannot be used. When .*  is  inside
       capturing  parentheses that are the subject of a backreference elsewhere in the pattern, a
       match at the start may fail where a later one succeeds. Consider, for example:

       (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth character. For this reason,
       such a pattern is not implicitly anchored.

       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 may
       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 rest of the pattern to  match.  Sometimes  it  is  useful  to
       prevent  this,  either to change the nature of the match, or to cause it fail earlier than
       it otherwise might, when the author of the pattern knows there is no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to the subject line

       123456bar

       After matching all 6 digits and then failing to match "foo",  the  normal  action  of  the
       matcher  is to try again with only 5 digits matching the \d+ item, and then with 4, 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 we use atomic grouping 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 this 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 in order to
       make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

       Atomic  groups  in  general can of course contain arbitrarily 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 additional  +  character  following  a  quantifier.  Using  this
       notation, the previous example can be rewritten as

       \d++foo

       Note 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 the ungreedy option is ignored.
       They are a convenient notation for the simpler forms of atomic group. However, there is no
       difference  in  the  meaning  of  a possessive quantifier and the equivalent atomic group,
       though there may be a performance difference; possessive quantifiers  should  be  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 Sun's 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 because 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 very long time indeed. The pattern

       (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-digits,  or  digits
       enclosed  in  <>, followed by either ! 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 a large number of ways, and
       all have to be tried. (The example uses [!?] rather than a single character  at  the  end,
       because 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 this:

       ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.

BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than  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 less than 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. In other words, the parentheses that are referenced
       need not be to the left of the reference  for  numbers  less  than  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 because a sequence such as \50 is interpreted as a
       character defined in octal. See the subsection entitled  "Non-printing  characters"  above
       for  further  details  of  the  handling of digits following a backslash. 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 of avoiding the ambiguity inherent in the use of digits following a backslash
       is to use the \g escape sequence, which is a feature introduced in Perl 5.10. This  escape
       must  be  followed  by  an  unsigned  number  or a negative number, optionally enclosed in
       braces. These examples are all 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 this example:

       (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to  the  most  recently  started  capturing  subpattern
       before  \g,  that is, is it equivalent to \2. 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 together fragments that contain references within themselves.

       A back reference matches whatever actually matched the capturing subpattern in the current
       subject string, rather than anything matching the subpattern itself (see  "Subpatterns  as
       subroutines" below for a way of doing that). So the pattern

       (sens|respons)e and \1ibility

       matches  "sense  and  sensibility"  and  "response and responsibility", but not "sense and
       responsibility". If caseful matching is in force at the time of the  back  reference,  the
       case of letters is relevant. For example,

       ((?i)rah)\s+\1

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the original capturing
       subpattern is matched caselessly.

       There are several 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). Perl 5.10's unified back reference syntax, in which \g can be  used  for
       both  numeric  and named references, is also supported. We could rewrite the above example
       in any of 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 may appear in the  pattern  before  or  after  the
       reference.

       There  may be more than one back reference to the same subpattern. If a subpattern has not
       actually been used in a particular match, any back  references  to  it  always  fail.  For
       example, the pattern

       (a|(bc))\2

       always  fails  if  it  starts  to  match  "a"  rather than "bc". Because there may 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  the  extended  option  is
       set,  this  can  be  whitespace.  Otherwise an empty comment (see "Comments" below) can be
       used.

       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 pattern

       (a|b\1)+

       matches any number of "a"s and also  "aba",  "ababbaa"  etc.  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.

ASSERTIONS

       An assertion is a test on the characters following or preceding the current matching point
       that  does not actually consume any characters. The simple assertions coded as \b, \B, \A,
       \G, \Z, \z, ^ and $ are described above.

       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, and may not be repeated, because it
       makes no sense to assert the same thing several times. If any kind of  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  carried  out
       only for positive assertions, because it does not make sense for negative assertions.

       Lookahead assertions

       Lookahead  assertions  start  with  (?=  for  positive  assertions  and  (?!  for negative
       assertions. For example,

       \w+(?=;)

       matches a word followed by a semicolon, but does not include the semicolon in  the  match,
       and

       foo(?!bar)

       matches  any  occurrence  of "foo" that is not followed by "bar". Note 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, because 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 (?!) because an empty string always matches,  so  an  assertion  that
       requires there not to be an empty string must always fail.

       Lookbehind assertions

       Lookbehind  assertions  start  with  (?<=  for  positive  assertions and (?<! for negative
       assertions. For example,

       (?<!foo)bar

       does find an occurrence of "bar" that  is  not  preceded  by  "foo".  The  contents  of  a
       lookbehind assertion are restricted such that all the strings it matches must have a fixed
       length. However, if there are several top-level alternatives, they do not all have to have
       the same fixed length. Thus

       (?<=bullock|donkey)

       is permitted, but

       (?<!dogs?|cats?)

       causes  an  error  at  compile  time.  Branches  that  match  different length strings are
       permitted only at the top level of a lookbehind assertion. This is an  extension  compared
       with  Perl  (at  least  for  5.8), which requires all branches to match the same length of
       string. An assertion such as

       (?<=ab(c|de))

       is not permitted, because its single top-level branch can match two different lengths, but
       it is acceptable if rewritten to use two top-level branches:

       (?<=abc|abde)

       In  some  cases,  the  Perl  5.10  escape sequence \K (see above) can be used instead of a
       lookbehind assertion; this is not restricted to a fixed-length.

       The implementation of lookbehind assertions is, for each alternative, to temporarily  move
       the  current  position  back  by  the  fixed  length  and  then try to match. If there are
       insufficient characters before the current position, the assertion fails.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) to appear in
       lookbehind  assertions,  because  it  makes  it  impossible to calculate the length of the
       lookbehind. The \X and \R escapes, which can match different numbers of  bytes,  are  also
       not permitted.

       Possessive  quantifiers  can  be used in conjunction with lookbehind assertions to specify
       efficient matching at the end of the subject string. Consider a simple pattern such as

       abcd$

       when applied to a long string that does not match. Because matching proceeds from left  to
       right, PCRE will look for each "a" in the subject and then see if what follows matches the
       rest of the pattern. If the pattern is specified as

       ^.*abcd$

       the initial .* matches the entire string at first, but when this fails (because  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

       Several assertions (of any sort) may occur in succession. For example,

       (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that  are  not  "999".  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 doesn't match "123abcfoo". A pattern to do that is

       (?<=\d{3}...)(?<!999)foo

       This  time  the  first  assertion looks at the preceding six characters, checking 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,

       (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by
       "foo", while

       (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any  three  characters
       that are not "999".

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  previous  capturing  subpattern  matched  or  not.  The  two possible forms of
       conditional subpattern are

         * (?(condition)yes-pattern)

         * (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern  is  used;  otherwise  the  no-pattern  (if
       present)  is  used.  If there are more than two alternatives in the subpattern, a compile-
       time error occurs.

       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 the capturing subpattern of that number has  previously  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. In looping constructs it
       can also make sense to refer to subsequent groups with constructs such as (?(+2).

       Consider the following pattern, which contains non-significant whitespace to make it  more
       readable  (assume  the  extended  option)  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,  since  no-pattern  is  not  present,  the
       subpattern matches nothing. In other words,  this  pattern  matches  a  sequence  of  non-
       parentheses, optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one, you could use a relative reference:

       ...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. However, there is a possible ambiguity
       with this syntax, because subpattern names may consist  entirely  of  digits.  PCRE  looks
       first  for  a  named  subpattern;  if it cannot find one and the name consists entirely of
       digits, PCRE looks for a subpattern of that number, which must be greater than zero. Using
       subpattern names that consist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

       (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )

       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 the subpattern whose number or
       name is given. This condition does not check the entire recursion stack.

       At "top level", all these recursion test conditions  are  false.  Recursive  patterns  are
       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 may 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 could 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 a another group named "byte"
       is defined. This matches an individual component of an IPv4 address (a  number  less  than
       256).  When  matching takes place, this part of the pattern is skipped because DEFINE acts
       like a false condition.

       The rest of the 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 may be
       a positive or negative lookahead or lookbehind assertion.  Consider  this  pattern,  again
       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. In other words, 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

       The  sequence  (?#  marks  the  start  of  a comment that continues up to the next closing
       parenthesis. Nested parentheses are not permitted. The characters that make up  a  comment
       play no part in the pattern matching at all.

       If  the  extended  option  is  set,  an  unescaped  #  character outside a character class
       introduces a comment that continues to immediately after the next newline in the pattern.

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
       (amongst other things). It does this by interpolating Perl code in the expression  at  run
       time,  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;

       The (?p{...}) item interpolates Perl code at run time, 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  also  for  individual  subpattern
       recursion.  After  its  introduction  in  PCRE  and  Python,  this  kind  of recursion was
       introduced into Perl at release 5.10.

       A special item that consists of (? followed by a number greater than zero  and  a  closing
       parenthesis  is  a  recursive call of the subpattern of the given number, provided that it
       occurs inside that subpattern. (If not, it is a "subroutine" call, which is  described  in
       the next section.) The special item (?R) or (?0) is a recursive call of the entire regular
       expression.

       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 PCRE pattern solves the nested parentheses problem (assume the extended option 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.

       If this were part of a larger pattern, you would not want to recurse the  entire  pattern,
       so instead you could use this:

       ( \( ( (?>[^()]+) | (?1) )* \) )

       We  have  put  the  pattern  into  parentheses,  and caused the recursion to refer 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. (A Perl 5.10 feature.) Instead of (?1) in the
       pattern above you can write (?-2) to refer to the second most recently opened  parentheses
       preceding  the  recursion.  In other words, a negative number counts capturing parentheses
       leftwards from the point at which it is encountered.

       It is also possible to refer to subsequently opened  parentheses,  by  writing  references
       such  as (?+2). However, these cannot be recursive because the reference is not inside the
       parentheses that are referenced. They are always "subroutine" calls, as described  in  the
       next section.

       An  alternative  approach is to use named parentheses instead. The Perl syntax for this is
       (?&name); PCRE's earlier syntax (?P>name) is also supported. We could  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 been looking at contains nested unlimited
       repeats, and so the use of atomic grouping 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 yields "no match" quickly. However, if atomic grouping is not used, the match runs  for
       a  very  long time indeed because there are so many different ways the + and * repeats can
       carve up the subject, and all have to be tested before failure can be reported.

       At the end of a match, the values set for any capturing subpatterns  are  those  from  the
       outermost  level  of  the  recursion  at which the subpattern value is set. If the pattern
       above is matched against

       (ab(cd)ef)

       the value for the capturing parentheses is "ef", which is the last value taken on  at  the
       top level. If additional parentheses are added, giving

         \( ( ( (?>[^()]+) | (?R) )* ) \)
            ^                        ^
            ^                        ^

       the string they capture is "ab(cd)ef", the contents of the top level parentheses.

       Do  not  confuse the (?R) item with the condition (R), which tests for recursion. Consider
       this pattern, which matches text in angle brackets, allowing for arbitrary  nesting.  Only
       digits  are  allowed  in nested brackets (that is, when recursing), whereas any characters
       are permitted at the outer level.

       < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start  of  a  conditional  subpattern,  with  two  different
       alternatives  for  the  recursive  and  non-recursive  cases.  The (?R) item is the actual
       recursive call.

SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (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 may be defined before or after the reference. A numbered
       reference can be absolute or relative, as in these examples:

         * (...(absolute)...)...(?2)...

         * (...(relative)...)...(?-1)...

         * (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

       (sens|respons)e and \1ibility

       matches  "sense  and  sensibility"  and  "response and responsibility", but not "sense and
       responsibility". If instead the pattern

       (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility"  as  well  as  the  other  two  strings.
       Another example is given in the discussion of DEFINE above.

       Like recursive subpatterns, a "subroutine" 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.

       When  a  subpattern  is used as a subroutine, processing options such as case-independence
       are fixed when the subpattern is defined. They cannot be changed for different calls.  For
       example, consider this pattern:

       (abc)(?i:(?-1))

       It  matches  "abcabc".  It does not match "abcABC" because the change of processing option
       does not affect the called subpattern.

BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are 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. In Perl, they are generally of the form (*VERB:ARG) but PCRE does
       not support the use of arguments, so its general form is just (*VERB). Any number of these
       verbs may occur in a pattern. There are two kinds:

       Verbs that act immediately

       The following verbs act as soon as they are encountered:

       (*ACCEPT)

       This  verb  causes  the  match to end successfully, skipping the remainder of the pattern.
       When inside a recursion, only the innermost pattern is  ended  immediately.  PCRE  differs
       from  Perl  in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the
       data so far is captured: in PCRE no data is captured. For example:

       A(A|B(*ACCEPT)|C)D

       This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is captured.

       (*FAIL) or (*F)

       This verb causes the match to fail, forcing backtracking to occur.  It  is  equivalent  to
       (?!) but easier to read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in
       PCRE. The nearest equivalent is the callout feature, as for example in this pattern:

       a+(?C)(*FAIL)

       A  match  with  the  string  "aaaa"  always  fails,  but  the callout is taken before each
       backtrack happens (in this example, 10 times).

       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, a failure is forced. The verbs differ in
       exactly what kind of failure occurs.

       (*COMMIT)

       This verb causes the whole match to fail outright if the rest  of  the  pattern  does  not
       match. Even if the pattern is unanchored, no further attempts to find a match by advancing
       the start point take place. Once (*COMMIT) has  been  passed,  re:run/3  is  committed  to
       finding 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."

       (*PRUNE)

       This verb causes the match to fail at the current position if the rest of the pattern does
       not  match.  If  the  pattern  is  unanchored,  the normal "bumpalong" advance to the next
       starting character then happens. Backtracking can occur as usual to the left of  (*PRUNE),
       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.

       (*SKIP)

       This verb 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) 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".
       Note  that a possessive quantifier does not have the same effect in 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".

       (*THEN)

       This verb causes a skip to the next alternation if the rest of the pattern does not match.
       That is, it cancels pending backtracking, but only within  the  current  alternation.  Its
       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  (*THEN)  is  used  outside  of  any
       alternation, it acts exactly like (*PRUNE).