Provided by: libpcre3-dev_8.31-2ubuntu2.3_amd64 
NAME
PCRE - Perl-compatible regular expressions
#include <pcre.h>
PCRE NATIVE API BASIC FUNCTIONS
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
void pcre_free_study(pcre_extra *extra);
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
PCRE NATIVE API STRING EXTRACTION FUNCTIONS
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
void pcre_free_substring(const char *stringptr);
void pcre_free_substring_list(const char **stringptr);
PCRE NATIVE API AUXILIARY FUNCTIONS
pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
void pcre_jit_stack_free(pcre_jit_stack *stack);
void pcre_assign_jit_stack(pcre_extra *extra,
pcre_jit_callback callback, void *data);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
int pcre_refcount(pcre *code, int adjust);
int pcre_config(int what, void *where);
const char *pcre_version(void);
int pcre_pattern_to_host_byte_order(pcre *code,
pcre_extra *extra, const unsigned char *tables);
PCRE NATIVE API INDIRECTED FUNCTIONS
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
void *(*pcre_stack_malloc)(size_t);
void (*pcre_stack_free)(void *);
int (*pcre_callout)(pcre_callout_block *);
PCRE 8-BIT AND 16-BIT LIBRARIES
From release 8.30, PCRE can be compiled as a library for handling 16-bit character strings as well as, or
instead of, the original library that handles 8-bit character strings. To avoid too much complication,
this document describes the 8-bit versions of the functions, with only occasional references to the
16-bit library.
The 16-bit functions operate in the same way as their 8-bit counterparts; they just use different data
types for their arguments and results, and their names start with pcre16_ instead of pcre_. For every
option that has UTF8 in its name (for example, PCRE_UTF8), there is a corresponding 16-bit name with UTF8
replaced by UTF16. This facility is in fact just cosmetic; the 16-bit option names define the same bit
values.
References to bytes and UTF-8 in this document should be read as references to 16-bit data quantities and
UTF-16 when using the 16-bit library, unless specified otherwise. More details of the specific
differences for the 16-bit library are given in the pcre16 page.
PCRE API OVERVIEW
PCRE has its own native API, which is described in this document. There are also some wrapper functions
(for the 8-bit library only) that correspond to the POSIX regular expression API, but they do not give
access to all the functionality. They are described in the pcreposix documentation. Both of these APIs
define a set of C function calls. A C++ wrapper (again for the 8-bit library only) is also distributed
with PCRE. It is documented in the pcrecpp page.
The native API C function prototypes are defined in the header file pcre.h, and on Unix-like systems the
(8-bit) library itself is called libpcre. It can normally be accessed by adding -lpcre to the command for
linking an application that uses PCRE. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
contain the major and minor release numbers for the library. Applications can use these to include
support for different releases of PCRE.
In a Windows environment, if you want to statically link an application program against a non-dll pcre.a
file, you must define PCRE_STATIC before including pcre.h or pcrecpp.h, because otherwise the
pcre_malloc() and pcre_free() exported functions will be declared __declspec(dllimport), with unwanted
results.
The functions pcre_compile(), pcre_compile2(), pcre_study(), and pcre_exec() are used for compiling and
matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the simplest
way of using them is provided in the file called pcredemo.c in the PCRE source distribution. A listing of
this program is given in the pcredemo documentation, and the pcresample documentation describes how to
compile and run it.
Just-in-time compiler support is an optional feature of PCRE that can be built in appropriate hardware
environments. It greatly speeds up the matching performance of many patterns. Simple programs can easily
request that it be used if available, by setting an option that is ignored when it is not relevant. More
complicated programs might need to make use of the functions pcre_jit_stack_alloc(),
pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control the JIT code's memory usage.
These functions are discussed in the pcrejit documentation.
A second matching function, pcre_dfa_exec(), which is not Perl-compatible, is also provided. This uses a
different algorithm for the matching. The alternative algorithm finds all possible matches (at a given
point in the subject), and scans the subject just once (unless there are lookbehind assertions). However,
this algorithm does not return captured substrings. A description of the two matching algorithms and
their advantages and disadvantages is given in the pcrematching documentation.
In addition to the main compiling and matching functions, there are convenience functions for extracting
captured substrings from a subject string that is matched by pcre_exec(). They are:
pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_get_stringnumber()
pcre_get_stringtable_entries()
pcre_free_substring() and pcre_free_substring_list() are also provided, to free the memory used for
extracted strings.
The function pcre_maketables() is used to build a set of character tables in the current locale for
passing to pcre_compile(), pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is provided
for specialist use. Most commonly, no special tables are passed, in which case internal tables that are
generated when PCRE is built are used.
The function pcre_fullinfo() is used to find out information about a compiled pattern. The function
pcre_version() returns a pointer to a string containing the version of PCRE and its date of release.
The function pcre_refcount() maintains a reference count in a data block containing a compiled pattern.
This is provided for the benefit of object-oriented applications.
The global variables pcre_malloc and pcre_free initially contain the entry points of the standard
malloc() and free() functions, respectively. PCRE calls the memory management functions via these
variables, so a calling program can replace them if it wishes to intercept the calls. This should be done
before calling any PCRE functions.
The global variables pcre_stack_malloc and pcre_stack_free are also indirections to memory management
functions. These special functions are used only when PCRE is compiled to use the heap for remembering
data, instead of recursive function calls, when running the pcre_exec() function. See the pcrebuild
documentation for details of how to do this. It is a non-standard way of building PCRE, for use in
environments that have limited stacks. Because of the greater use of memory management, it runs more
slowly. Separate functions are provided so that special-purpose external code can be used for this case.
When used, these functions are always called in a stack-like manner (last obtained, first freed), and
always for memory blocks of the same size. There is a discussion about PCRE's stack usage in the
pcrestack documentation.
The global variable pcre_callout initially contains NULL. It can be set by the caller to a "callout"
function, which PCRE will then call at specified points during a matching operation. Details are given in
the pcrecallout documentation.
NEWLINES
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. The Unicode newline sequences are the three just mentioned,
plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS
(line separator, U+2028), and PS (paragraph separator, U+2029).
Each of the first three conventions is used by at least one operating system as its standard newline
sequence. When PCRE is built, a default can be specified. The default default is LF, which is the Unix
standard. When PCRE is run, the default can be overridden, either when a pattern is compiled, or when it
is matched.
At compile time, the newline convention can be specified by the options argument of pcre_compile(), or it
can be specified by special text at the start of the pattern itself; this overrides any other settings.
See the pcrepattern page for details of the special character sequences.
In the PCRE documentation the word "newline" is used to mean "the character or pair of characters that
indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and
dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending
sequence, the match position advancement for a non-anchored pattern. There is more detail about this in
the section on pcre_exec() options below.
The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor
does it affect what \R matches, which is controlled in a similar way, but by separate options.
MULTITHREADING
The PCRE functions can be used in multi-threading applications, with the proviso that the memory
management functions pointed to by pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and
the callout function pointed to by pcre_callout, are shared by all threads.
The compiled form of a regular expression is not altered during matching, so the same compiled pattern
can safely be used by several threads at once.
If the just-in-time optimization feature is being used, it needs separate memory stack areas for each
thread. See the pcrejit documentation for more details.
SAVING PRECOMPILED PATTERNS FOR LATER USE
The compiled form of a regular expression can be saved and re-used at a later time, possibly by a
different program, and even on a host other than the one on which it was compiled. Details are given in
the pcreprecompile documentation, which includes a description of the pcre_pattern_to_host_byte_order()
function. However, compiling a regular expression with one version of PCRE for use with a different
version is not guaranteed to work and may cause crashes.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE client to discover which optional features have
been compiled into the PCRE library. The pcrebuild documentation has more details about these optional
features.
The first argument for pcre_config() is an integer, specifying which information is required; the second
argument is a pointer to a variable into which the information is placed. The returned value is zero on
success, or the negative error code PCRE_ERROR_BADOPTION if the value in the first argument is not
recognized. The following information is available:
PCRE_CONFIG_UTF8
The output is an integer that is set to one if UTF-8 support is available; otherwise it is set to zero.
If this option is given to the 16-bit version of this function, pcre16_config(), the result is
PCRE_ERROR_BADOPTION.
PCRE_CONFIG_UTF16
The output is an integer that is set to one if UTF-16 support is available; otherwise it is set to zero.
This value should normally be given to the 16-bit version of this function, pcre16_config(). If it is
given to the 8-bit version of this function, the result is PCRE_ERROR_BADOPTION.
PCRE_CONFIG_UNICODE_PROPERTIES
The output is an integer that is set to one if support for Unicode character properties is available;
otherwise it is set to zero.
PCRE_CONFIG_JIT
The output is an integer that is set to one if support for just-in-time compiling is available; otherwise
it is set to zero.
PCRE_CONFIG_JITTARGET
The output is a pointer to a zero-terminated "const char *" string. If JIT support is available, the
string contains the name of the architecture for which the JIT compiler is configured, for example "x86
32bit (little endian + unaligned)". If JIT support is not available, the result is NULL.
PCRE_CONFIG_NEWLINE
The output is an integer whose value specifies the default character sequence that is recognized as
meaning "newline". The four values that are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for
ANYCRLF, and -1 for ANY. Though they are derived from ASCII, the same values are returned in EBCDIC
environments. The default should normally correspond to the standard sequence for your operating system.
PCRE_CONFIG_BSR
The output is an integer whose value indicates what character sequences the \R escape sequence matches by
default. A value of 0 means that \R matches any Unicode line ending sequence; a value of 1 means that \R
matches only CR, LF, or CRLF. The default can be overridden when a pattern is compiled or matched.
PCRE_CONFIG_LINK_SIZE
The output is an integer that contains the number of bytes used for internal linkage in compiled regular
expressions. For the 8-bit library, the value can be 2, 3, or 4. For the 16-bit library, the value is
either 2 or 4 and is still a number of bytes. The default value of 2 is sufficient for all but the most
massive patterns, since it allows the compiled pattern to be up to 64K in size. Larger values allow
larger regular expressions to be compiled, at the expense of slower matching.
PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
The output is an integer that contains the threshold above which the POSIX interface uses malloc() for
output vectors. Further details are given in the pcreposix documentation.
PCRE_CONFIG_MATCH_LIMIT
The output is a long integer that gives the default limit for the number of internal matching function
calls in a pcre_exec() execution. Further details are given with pcre_exec() below.
PCRE_CONFIG_MATCH_LIMIT_RECURSION
The output is a long integer that gives the default limit for the depth of recursion when calling the
internal matching function in a pcre_exec() execution. Further details are given with pcre_exec() below.
PCRE_CONFIG_STACKRECURSE
The output is an integer that is set to one if internal recursion when running pcre_exec() is implemented
by recursive function calls that use the stack to remember their state. This is the usual way that PCRE
is compiled. The output is zero if PCRE was compiled to use blocks of data on the heap instead of
recursive function calls. In this case, pcre_stack_malloc and pcre_stack_free are called to manage memory
blocks on the heap, thus avoiding the use of the stack.
COMPILING A PATTERN
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
Either of the functions pcre_compile() or pcre_compile2() can be called to compile a pattern into an
internal form. The only difference between the two interfaces is that pcre_compile2() has an additional
argument, errorcodeptr, via which a numerical error code can be returned. To avoid too much repetition,
we refer just to pcre_compile() below, but the information applies equally to pcre_compile2().
The pattern is a C string terminated by a binary zero, and is passed in the pattern argument. A pointer
to a single block of memory that is obtained via pcre_malloc is returned. This contains the compiled code
and related data. The pcre type is defined for the returned block; this is a typedef for a structure
whose contents are not externally defined. It is up to the caller to free the memory (via pcre_free) when
it is no longer required.
Although the compiled code of a PCRE regex is relocatable, that is, it does not depend on memory
location, the complete pcre data block is not fully relocatable, because it may contain a copy of the
tableptr argument, which is an address (see below).
The options argument contains various bit settings that affect the compilation. It should be zero if no
options are required. The available options are described below. Some of them (in particular, those that
are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see
the detailed description in the pcrepattern documentation). For those options that can be different in
different parts of the pattern, the contents of the options argument specifies their settings at the
start of compilation and execution. The PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx,
PCRE_NO_UTF8_CHECK, and PCRE_NO_START_OPTIMIZE options can be set at the time of matching as well as at
compile time.
If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern
fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual
error message. This is a static string that is part of the library. You must not try to free it.
Normally, the offset from the start of the pattern to the byte that was being processed when the error
was discovered is placed in the variable pointed to by erroffset, which must not be NULL (if it is, an
immediate error is given). However, for an invalid UTF-8 string, the offset is that of the first byte of
the failing character.
Some errors are not detected until the whole pattern has been scanned; in these cases, the offset passed
back is the length of the pattern. Note that the offset is in bytes, not characters, even in UTF-8 mode.
It may sometimes point into the middle of a UTF-8 character.
If pcre_compile2() is used instead of pcre_compile(), and the errorcodeptr argument is not NULL, a non-
zero error code number is returned via this argument in the event of an error. This is in addition to the
textual error message. Error codes and messages are listed below.
If the final argument, tableptr, is NULL, PCRE uses a default set of character tables that are built when
PCRE is compiled, using the default C locale. Otherwise, tableptr must be an address that is the result
of a call to pcre_maketables(). This value is stored with the compiled pattern, and used again by
pcre_exec(), unless another table pointer is passed to it. For more discussion, see the section on locale
support below.
This code fragment shows a typical straightforward call to pcre_compile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
The following names for option bits are defined in the pcre.h header file:
PCRE_ANCHORED
If this bit is set, 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, which is the only way to do it in Perl.
PCRE_AUTO_CALLOUT
If this bit is set, pcre_compile() automatically inserts callout items, all with number 255, before each
pattern item. For discussion of the callout facility, see the pcrecallout documentation.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is
either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. The default is specified
when PCRE is built. It can be overridden from within the pattern, or by setting an option when a compiled
pattern is matched.
PCRE_CASELESS
If this bit is set, 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. 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.
PCRE_DOLLAR_ENDONLY
If this bit is set, 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 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. There is
no equivalent to this option in Perl, and no way to set it within a pattern.
PCRE_DOTALL
If this bit is set, a dot metacharacter in the pattern matches a character of any value, including one
that indicates a newline. However, it only ever matches one character, even if newlines are coded as
CRLF. Without this option, 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 the setting of this option.
PCRE_DUPNAMES
If this bit is set, 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; see also the pcrepattern documentation.
PCRE_EXTENDED
If this bit is set, white space data characters in the pattern are totally ignored except when escaped or
inside a character class. White space does not include the VT character (code 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.
Which characters are interpreted as newlines is controlled by the options passed to pcre_compile() or by
a special sequence at the start of the pattern, as described in the section entitled "Newline
conventions" in the pcrepattern documentation. Note that the end of this type of comment is a literal
newline sequence in the pattern; escape sequences that happen to represent a newline do not count.
This option makes it possible to include comments inside complicated patterns. Note, however, that this
applies only to data characters. White space characters may never appear within special character
sequences in a pattern, for example within the sequence (?( that introduces a conditional subpattern.
PCRE_EXTRA
This option was invented in order to turn on additional functionality of PCRE that is incompatible with
Perl, but it is currently of very little use. When set, any backslash in a pattern that is followed by a
letter that has no special meaning causes an error, thus reserving these combinations for future
expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as
a literal. (Perl can, however, be persuaded to give an error for this, by running it with the -w option.)
There are at present no other features controlled by this option. It can also be set by a (?X) option
setting within a pattern.
PCRE_FIRSTLINE
If this option is set, 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.
PCRE_JAVASCRIPT_COMPAT
If this option is set, PCRE's behaviour is changed in some ways so that it is compatible with JavaScript
rather than Perl. The changes are as follows:
(1) A lone closing square bracket in a pattern causes a compile-time error, because this is illegal in
JavaScript (by default it is treated as a data character). Thus, the pattern AB]CD becomes illegal when
this option is set.
(2) At run time, a back reference to an unset subpattern group matches an empty string (by default this
causes the current matching alternative to fail). A pattern such as (\1)(a) succeeds when this option is
set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility.
(3) \U matches an upper case "U" character; by default \U causes a compile time error (Perl uses \U to
upper case subsequent characters).
(4) \u matches a lower case "u" character unless it is followed by four hexadecimal digits, in which case
the hexadecimal number defines the code point to match. By default, \u causes a compile time error (Perl
uses it to upper case the following character).
(5) \x matches a lower case "x" character unless it is followed by two hexadecimal digits, in which case
the hexadecimal number defines the code point to match. By default, as in Perl, a hexadecimal number is
always expected after \x, but it may have zero, one, or two digits (so, for example, \xz matches a binary
zero character followed by z).
PCRE_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 PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.
When PCRE_MULTILINE it is set, 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 PCRE_MULTILINE has no effect.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the default newline definition that was chosen when PCRE was built. Setting the
first or the second specifies that a newline is indicated by a single character (CR or LF, respectively).
Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the two-character CRLF sequence.
Setting PCRE_NEWLINE_ANYCRLF specifies that any of the three preceding sequences should be recognized.
Setting PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be recognized. The Unicode
newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF
(form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator,
U+2029). For the 8-bit library, the last two are recognized only in UTF-8 mode.
The newline setting in the options word uses three bits that are treated as a number, giving eight
possibilities. Currently only six are used (default plus the five values above). This means that if you
set more than one newline option, the combination may or may not be sensible. For example,
PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but other combinations may yield
unused numbers and cause an error.
The only time that a line break in a pattern is specially recognized when compiling is when PCRE_EXTENDED
is set. CR and LF are white space characters, and so are ignored in this mode. Also, an unescaped #
outside a character class indicates a comment that lasts until after the next line break sequence. In
other circumstances, line break sequences in patterns are treated as literal data.
The newline option that is set at compile time becomes the default that is used for pcre_exec() and
pcre_dfa_exec(), but it can be overridden.
PCRE_NO_AUTO_CAPTURE
If this option is set, it 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.
NO_START_OPTIMIZE
This is an option that acts at matching time; that is, it is really an option for pcre_exec() or
pcre_dfa_exec(). If it is set at compile time, it is remembered with the compiled pattern and assumed at
matching time. For details see the discussion of PCRE_NO_START_OPTIMIZE below.
PCRE_UCP
This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX
character classes. By default, only ASCII characters are recognized, but if PCRE_UCP is set, Unicode
properties are used instead to classify characters. More details are given in the section on generic
character types in the pcrepattern page. If you set PCRE_UCP, matching one of the items it affects takes
much longer. The option is available only if PCRE has been compiled with Unicode property support.
PCRE_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.
PCRE_UTF8
This option causes PCRE to regard both the pattern and the subject as strings of UTF-8 characters instead
of single-byte strings. However, it is available only when PCRE is built to include UTF support. If not,
the use of this option provokes an error. Details of how this option changes the behaviour of PCRE are
given in the pcreunicode page.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is automatically checked. There is a
discussion about the validity of UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
found, pcre_compile() returns an error. If you already know that your pattern is valid, and you want to
skip this check for performance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
effect of passing an invalid UTF-8 string as a pattern is undefined. It may cause your program to crash.
Note that this option can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity
checking of subject strings.
COMPILATION ERROR CODES
The following table lists the error codes than may be returned by pcre_compile2(), along with the error
messages that may be returned by both compiling functions. Note that error messages are always 8-bit
ASCII strings, even in 16-bit mode. As PCRE has developed, some error codes have fallen out of use. To
avoid confusion, they have not been re-used.
0 no error
1 \ at end of pattern
2 \c at end of pattern
3 unrecognized character follows \
4 numbers out of order in {} quantifier
5 number too big in {} quantifier
6 missing terminating ] for character class
7 invalid escape sequence in character class
8 range out of order in character class
9 nothing to repeat
10 [this code is not in use]
11 internal error: unexpected repeat
12 unrecognized character after (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NULL
17 unknown option bit(s) set
18 missing ) after comment
19 [this code is not in use]
20 regular expression is too large
21 failed to get memory
22 unmatched parentheses
23 internal error: code overflow
24 unrecognized character after (?<
25 lookbehind assertion is not fixed length
26 malformed number or name after (?(
27 conditional group contains more than two branches
28 assertion expected after (?(
29 (?R or (?[+-]digits must be followed by )
30 unknown POSIX class name
31 POSIX collating elements are not supported
32 this version of PCRE is compiled without UTF support
33 [this code is not in use]
34 character value in \x{...} sequence is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N{name}, \U, or \u
38 number after (?C is > 255
39 closing ) for (?C expected
40 recursive call could loop indefinitely
41 unrecognized character after (?P
42 syntax error in subpattern name (missing terminator)
43 two named subpatterns have the same name
44 invalid UTF-8 string (specifically UTF-8)
45 support for \P, \p, and \X has not been compiled
46 malformed \P or \p sequence
47 unknown property name after \P or \p
48 subpattern name is too long (maximum 32 characters)
49 too many named subpatterns (maximum 10000)
50 [this code is not in use]
51 octal value is greater than \377 in 8-bit non-UTF-8 mode
52 internal error: overran compiling workspace
53 internal error: previously-checked referenced subpattern
not found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
60 (*VERB) not recognized
61 number is too big
62 subpattern name expected
63 digit expected after (?+
64 ] is an invalid data character in JavaScript compatibility mode
65 different names for subpatterns of the same number are
not allowed
66 (*MARK) must have an argument
67 this version of PCRE is not compiled with Unicode property
support
68 \c must be followed by an ASCII character
69 \k is not followed by a braced, angle-bracketed, or quoted name
70 internal error: unknown opcode in find_fixedlength()
71 \N is not supported in a class
72 too many forward references
73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
74 invalid UTF-16 string (specifically UTF-16)
75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
76 character value in \u.... sequence is too large
The numbers 32 and 10000 in errors 48 and 49 are defaults; different values may be used if the limits
were changed when PCRE was built.
STUDYING A PATTERN
pcre_extra *pcre_study(const pcre *code, int options
const char **errptr);
If a compiled pattern is going to be used several times, it is worth spending more time analyzing it in
order to speed up the time taken for matching. The function pcre_study() takes a pointer to a compiled
pattern as its first argument. If studying the pattern produces additional information that will help
speed up matching, pcre_study() returns a pointer to a pcre_extra block, in which the study_data field
points to the results of the study.
The returned value from pcre_study() can be passed directly to pcre_exec() or pcre_dfa_exec(). However, a
pcre_extra block also contains other fields that can be set by the caller before the block is passed;
these are described below in the section on matching a pattern.
If studying the pattern does not produce any useful information, pcre_study() returns NULL. In that
circumstance, if the calling program wants to pass any of the other fields to pcre_exec() or
pcre_dfa_exec(), it must set up its own pcre_extra block.
The second argument of pcre_study() contains option bits. There are three options:
PCRE_STUDY_JIT_COMPILE
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
If any of these are set, and the just-in-time compiler is available, the pattern is further compiled into
machine code that executes much faster than the pcre_exec() interpretive matching function. If the just-
in-time compiler is not available, these options are ignored. All other bits in the options argument must
be zero.
JIT compilation is a heavyweight optimization. It can take some time for patterns to be analyzed, and for
one-off matches and simple patterns the benefit of faster execution might be offset by a much slower
study time. Not all patterns can be optimized by the JIT compiler. For those that cannot be handled,
matching automatically falls back to the pcre_exec() interpreter. For more details, see the pcrejit
documentation.
The third argument for pcre_study() is a pointer for an error message. If studying succeeds (even if no
data is returned), the variable it points to is set to NULL. Otherwise it is set to point to a textual
error message. This is a static string that is part of the library. You must not try to free it. You
should test the error pointer for NULL after calling pcre_study(), to be sure that it has run
successfully.
When you are finished with a pattern, you can free the memory used for the study data by calling
pcre_free_study(). This function was added to the API for release 8.20. For earlier versions, the memory
could be freed with pcre_free(), just like the pattern itself. This will still work in cases where JIT
optimization is not used, but it is advisable to change to the new function when convenient.
This is a typical way in which pcre_study() is used (except that in a real application there should be
tests for errors):
int rc;
pcre *re;
pcre_extra *sd;
re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
sd = pcre_study(
re, /* result of pcre_compile() */
0, /* no options */
&error); /* set to NULL or points to a message */
rc = pcre_exec( /* see below for details of pcre_exec() options */
re, sd, "subject", 7, 0, 0, ovector, 30);
...
pcre_free_study(sd);
pcre_free(re);
Studying a pattern does two things: first, a lower bound for the length of subject string that is needed
to match the pattern is computed. This does not mean that there are any strings of that length that
match, but it does guarantee that no shorter strings match. The value is used by pcre_exec() and
pcre_dfa_exec() to avoid wasting time by trying to match strings that are shorter than the lower bound.
You can find out the value in a calling program via the pcre_fullinfo() function.
Studying a pattern is also useful for non-anchored patterns that do not have a single fixed starting
character. A bitmap of possible starting bytes is created. This speeds up finding a position in the
subject at which to start matching. (In 16-bit mode, the bitmap is used for 16-bit values less than 256.)
These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), and the information is also used
by the JIT compiler. The optimizations can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execution is also disabled. You might
want to do this if your pattern contains callouts or (*MARK) and you want to make use of these facilities
in cases where matching fails. See the discussion of PCRE_NO_START_OPTIMIZE below.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by
reference to a set of tables, indexed by character value. When running in UTF-8 mode, this applies only
to characters with codes less than 128. By default, higher-valued codes never match escapes such as \w or
\d, but they can be tested with \p if PCRE is built with Unicode character property support.
Alternatively, the PCRE_UCP option can be set at compile time; this causes \w and friends to use Unicode
property support instead of built-in tables. The use of locales with Unicode is discouraged. If you are
handling characters with codes greater than 128, you should either use UTF-8 and Unicode, or use locales,
but not try to mix the two.
PCRE contains an internal set of tables that are used when the final argument of pcre_compile() is NULL.
These are sufficient for many applications. Normally, the internal tables recognize only ASCII
characters. However, when PCRE is built, it is possible to cause the internal tables to be rebuilt in the
default "C" locale of the local system, which may cause them to be different.
The internal tables can always be overridden by tables supplied by the application that calls PCRE. These
may be created in a different locale from the default. As more and more applications change to using
Unicode, the need for this locale support is expected to die away.
External tables are built by calling the pcre_maketables() function, which has no arguments, in the
relevant locale. The result can then be passed to pcre_compile() or pcre_exec() as often as necessary.
For example, to build and use tables that are appropriate for the French locale (where accented
characters with values greater than 128 are treated as letters), the following code could be used:
setlocale(LC_CTYPE, "fr_FR");
tables = pcre_maketables();
re = pcre_compile(..., tables);
The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name
for the French locale is "french".
When pcre_maketables() runs, the tables are built in memory that is obtained via pcre_malloc. It is the
caller's responsibility to ensure that the memory containing the tables remains available for as long as
it is needed.
The pointer that is passed to pcre_compile() is saved with the compiled pattern, and the same tables are
used via this pointer by pcre_study() and normally also by pcre_exec(). Thus, by default, for any single
pattern, compilation, studying and matching all happen in the same locale, but different patterns can be
compiled in different locales.
It is possible to pass a table pointer or NULL (indicating the use of the internal tables) to
pcre_exec(). Although not intended for this purpose, this facility could be used to match a pattern in a
different locale from the one in which it was compiled. Passing table pointers at run time is discussed
below in the section on matching a pattern.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
The pcre_fullinfo() function returns information about a compiled pattern. It replaces the pcre_info()
function, which was removed from the library at version 8.30, after more than 10 years of obsolescence.
The first argument for pcre_fullinfo() is a pointer to the compiled pattern. The second argument is the
result of pcre_study(), or NULL if the pattern was not studied. The third argument specifies which piece
of information is required, and the fourth argument is a pointer to a variable to receive the data. The
yield of the function is zero for success, or one of the following negative numbers:
PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
endianness
PCRE_ERROR_BADOPTION the value of what was invalid
The "magic number" is placed at the start of each compiled pattern as an simple check against passing an
arbitrary memory pointer. The endianness error can occur if a compiled pattern is saved and reloaded on a
different host. Here is a typical call of pcre_fullinfo(), to obtain the length of the compiled pattern:
int rc;
size_t length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
sd, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length); /* where to put the data */
The possible values for the third argument are defined in pcre.h, and are as follows:
PCRE_INFO_BACKREFMAX
Return the number of the highest back reference in the pattern. The fourth argument should point to an
int variable. Zero is returned if there are no back references.
PCRE_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern. The fourth argument should point to an int
variable.
PCRE_INFO_DEFAULT_TABLES
Return a pointer to the internal default character tables within PCRE. The fourth argument should point
to an unsigned char * variable. This information call is provided for internal use by the pcre_study()
function. External callers can cause PCRE to use its internal tables by passing a NULL table pointer.
PCRE_INFO_FIRSTBYTE
Return information about the first data unit of any matched string, for a non-anchored pattern. (The name
of this option refers to the 8-bit library, where data units are bytes.) The fourth argument should point
to an int variable.
If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), its
value is returned. In the 8-bit library, the value is always less than 256; in the 16-bit library the
value can be up to 0xffff.
If there is no fixed first value, and if either
(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or
(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern
would be anchored),
-1 is returned, indicating that the pattern matches only at the start of a subject string or after any
newline within the string. Otherwise -2 is returned. For anchored patterns, -2 is returned.
PCRE_INFO_FIRSTTABLE
If the pattern was studied, and this resulted in the construction of a 256-bit table indicating a fixed
set of values for the first data unit in any matching string, a pointer to the table is returned.
Otherwise NULL is returned. The fourth argument should point to an unsigned char * variable.
PCRE_INFO_HASCRORLF
Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The fourth
argument should point to an int variable. An explicit match is either a literal CR or LF character, or \r
or \n.
PCRE_INFO_JCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The fourth argument
should point to an int variable. (?J) and (?-J) set and unset the local PCRE_DUPNAMES option,
respectively.
PCRE_INFO_JIT
Return 1 if the pattern was studied with one of the JIT options, and just-in-time compiling was
successful. The fourth argument should point to an int variable. A return value of 0 means that JIT
support is not available in this version of PCRE, or that the pattern was not studied with a JIT option,
or that the JIT compiler could not handle this particular pattern. See the pcrejit documentation for
details of what can and cannot be handled.
PCRE_INFO_JITSIZE
If the pattern was successfully studied with a JIT option, return the size of the JIT compiled code,
otherwise return zero. The fourth argument should point to a size_t variable.
PCRE_INFO_LASTLITERAL
Return the value of the rightmost literal data unit that must exist in any matched string, other than at
its start, if such a value has been recorded. The fourth argument should point to an int variable. If
there is no such value, -1 is returned. For anchored patterns, a last literal value is recorded only if
it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is
"z", but for /^a\dz\d/ the returned value is -1.
PCRE_INFO_MAXLOOKBEHIND
Return the number of characters (NB not bytes) in the longest lookbehind assertion in the pattern. Note
that the simple assertions \b and \B require a one-character lookbehind. This information is useful when
doing multi-segment matching using the partial matching facilities.
PCRE_INFO_MINLENGTH
If the pattern was studied and a minimum length for matching subject strings was computed, its value is
returned. Otherwise the returned value is -1. The value is a number of characters, which in UTF-8 mode
may be different from the number of bytes. The fourth argument should point to an int variable. A non-
negative value is a lower bound to the length of any matching string. There may not be any strings of
that length that do actually match, but every string that does match is at least that long.
PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE
PCRE supports the use of named as well as numbered capturing parentheses. The names are just an
additional way of identifying the parentheses, which still acquire numbers. Several convenience functions
such as pcre_get_named_substring() are provided for extracting captured substrings by name. It is also
possible to extract the data directly, by first converting the name to a number in order to access the
correct pointers in the output vector (described with pcre_exec() below). To do the conversion, you need
to use the name-to-number map, which is described by these three values.
The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT gives the number of entries, and
PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both of these return an int value. The entry size
depends on the length of the longest name. PCRE_INFO_NAMETABLE returns a pointer to the first entry of
the table. This is a pointer to char in the 8-bit library, where the first two bytes of each entry are
the number of the capturing parenthesis, most significant byte first. In the 16-bit library, the pointer
points to 16-bit data units, the first of which contains the parenthesis number. The rest of the entry is
the corresponding name, zero terminated.
The names are in alphabetical order. Duplicate names may appear if (?| is used to create multiple groups
with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern
page. Duplicate names for subpatterns with different numbers are permitted only if PCRE_DUPNAMES is set.
In all cases of duplicate names, they appear in the table in the order in which they were found in the
pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not
necessarily the case because later subpatterns may have lower numbers.
As a simple example of the name/number table, consider the following pattern after compilation by the
8-bit library (assume PCRE_EXTENDED is set, so white space - including newlines - is ignored):
(?<date> (?<year>(\d\d)?\d\d) -
(?<month>\d\d) - (?<day>\d\d) )
There are four named subpatterns, so the table has four entries, and each entry in the table is eight
bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes
shown as ??:
00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns using the name-to-number map, remember that the
length of the entries is likely to be different for each compiled pattern.
PCRE_INFO_OKPARTIAL
Return 1 if the pattern can be used for partial matching with pcre_exec(), otherwise 0. The fourth
argument should point to an int variable. From release 8.00, this always returns 1, because the
restrictions that previously applied to partial matching have been lifted. The pcrepartial documentation
gives details of partial matching.
PCRE_INFO_OPTIONS
Return a copy of the options with which the pattern was compiled. The fourth argument should point to an
unsigned long int variable. These option bits are those specified in the call to pcre_compile(), modified
by any top-level option settings at the start of the pattern itself. In other words, they are the options
that will be in force when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED.
A pattern is automatically anchored by PCRE if all of its top-level alternatives begin with one of the
following:
^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears
For such patterns, the PCRE_ANCHORED bit is set in the options returned by pcre_fullinfo().
PCRE_INFO_SIZE
Return the size of the compiled pattern in bytes (for both libraries). The fourth argument should point
to a size_t variable. This value does not include the size of the pcre structure that is returned by
pcre_compile(). The value that is passed as the argument to pcre_malloc() when pcre_compile() is getting
memory in which to place the compiled data is the value returned by this option plus the size of the pcre
structure. Studying a compiled pattern, with or without JIT, does not alter the value returned by this
option.
PCRE_INFO_STUDYSIZE
Return the size in bytes of the data block pointed to by the study_data field in a pcre_extra block. If
pcre_extra is NULL, or there is no study data, zero is returned. The fourth argument should point to a
size_t variable. The study_data field is set by pcre_study() to record information that will speed up
matching (see the section entitled "Studying a pattern" above). The format of the study_data block is
private, but its length is made available via this option so that it can be saved and restored (see the
pcreprecompile documentation for details).
REFERENCE COUNTS
int pcre_refcount(pcre *code, int adjust);
The pcre_refcount() function is used to maintain a reference count in the data block that contains a
compiled pattern. It is provided for the benefit of applications that operate in an object-oriented
manner, where different parts of the application may be using the same compiled pattern, but you want to
free the block when they are all done.
When a pattern is compiled, the reference count field is initialized to zero. It is changed only by
calling this function, whose action is to add the adjust value (which may be positive or negative) to it.
The yield of the function is the new value. However, the value of the count is constrained to lie between
0 and 65535, inclusive. If the new value is outside these limits, it is forced to the appropriate limit
value.
Except when it is zero, the reference count is not correctly preserved if a pattern is compiled on one
host and then transferred to a host whose byte-order is different. (This seems a highly unlikely
scenario.)
MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
The function pcre_exec() is called to match a subject string against a compiled pattern, which is passed
in the code argument. If the pattern was studied, the result of the study should be passed in the extra
argument. You can call pcre_exec() with the same code and extra arguments as many times as you like, in
order to match different subject strings with the same pattern.
This function is the main matching facility of the library, and it operates in a Perl-like manner. For
specialist use there is also an alternative matching function, which is described below in the section
about the pcre_dfa_exec() function.
In most applications, the pattern will have been compiled (and optionally studied) in the same process
that calls pcre_exec(). However, it is possible to save compiled patterns and study data, and then use
them later in different processes, possibly even on different hosts. For a discussion about this, see the
pcreprecompile documentation.
Here is an example of a simple call to pcre_exec():
int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
30); /* number of elements (NOT size in bytes) */
Extra data for pcre_exec()
If the extra argument is not NULL, it must point to a pcre_extra data block. The pcre_study() function
returns such a block (when it doesn't return NULL), but you can also create one for yourself, and pass
additional information in it. The pcre_extra block contains the following fields (not necessarily in this
order):
unsigned long int flags;
void *study_data;
void *executable_jit;
unsigned long int match_limit;
unsigned long int match_limit_recursion;
void *callout_data;
const unsigned char *tables;
unsigned char **mark;
In the 16-bit version of this structure, the mark field has type "PCRE_UCHAR16 **".
The flags field is used to specify which of the other fields are set. The flag bits are:
PCRE_EXTRA_CALLOUT_DATA
PCRE_EXTRA_EXECUTABLE_JIT
PCRE_EXTRA_MARK
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_MATCH_LIMIT_RECURSION
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_TABLES
Other flag bits should be set to zero. The study_data field and sometimes the executable_jit field are
set in the pcre_extra block that is returned by pcre_study(), together with the appropriate flag bits.
You should not set these yourself, but you may add to the block by setting other fields and their
corresponding flag bits.
The match_limit field provides a means of preventing PCRE from using up a vast amount of resources when
running patterns that are not going to match, but which have a very large number of possibilities in
their search trees. The classic example is a pattern that uses nested unlimited repeats.
Internally, pcre_exec() uses a function called match(), which it calls repeatedly (sometimes
recursively). The limit set by match_limit is imposed on the number of times this function is called
during a match, which has the effect of limiting the amount of backtracking that can take place. For
patterns that are not anchored, the count restarts from zero for each position in the subject string.
When pcre_exec() is called with a pattern that was successfully studied with a JIT option, the way that
the matching is executed is entirely different. However, there is still the possibility of runaway
matching that goes on for a very long time, and so the match_limit value is also used in this case (but
in a different way) to limit how long the matching can continue.
The default value for the limit can be set when PCRE is built; the default default is 10 million, which
handles all but the most extreme cases. You can override the default by suppling pcre_exec() with a
pcre_extra block in which match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If
the limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
The match_limit_recursion field is similar to match_limit, but instead of limiting the total number of
times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number
than the total number of calls, because not all calls to match() are recursive. This limit is of use
only if it is set smaller than match_limit.
Limiting the recursion depth limits the amount of machine stack that can be used, or, when PCRE has been
compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used. This
limit is not relevant, and is ignored, when matching is done using JIT compiled code.
The default value for match_limit_recursion can be set when PCRE is built; the default default is the
same value as the default for match_limit. You can override the default by suppling pcre_exec() with a
pcre_extra block in which match_limit_recursion is set, and PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in
the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
The callout_data field is used in conjunction with the "callout" feature, and is described in the
pcrecallout documentation.
The tables field is used to pass a character tables pointer to pcre_exec(); this overrides the value that
is stored with the compiled pattern. A non-NULL value is stored with the compiled pattern only if custom
tables were supplied to pcre_compile() via its tableptr argument. If NULL is passed to pcre_exec() using
this mechanism, it forces PCRE's internal tables to be used. This facility is helpful when re-using
patterns that have been saved after compiling with an external set of tables, because the external tables
might be at a different address when pcre_exec() is called. See the pcreprecompile documentation for a
discussion of saving compiled patterns for later use.
If PCRE_EXTRA_MARK is set in the flags field, the mark field must be set to point to a suitable variable.
If the pattern contains any backtracking control verbs such as (*MARK:NAME), and the execution ends up
with a name to pass back, a pointer to the name string (zero terminated) is placed in the variable
pointed to by the mark field. The names are within the compiled pattern; if you wish to retain such a
name you must copy it before freeing the memory of a compiled pattern. If there is no name to pass back,
the variable pointed to by the mark field is set to NULL. For details of the backtracking control verbs,
see the section entitled "Backtracking control" in the pcrepattern documentation.
Option bits for pcre_exec()
The unused bits of the options argument for pcre_exec() must be zero. The only bits that may be set are
PCRE_ANCHORED, PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
If the pattern was successfully studied with one of the just-in-time (JIT) compile options, the only
supported options for JIT execution are PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an unsupported option is used, JIT
execution is disabled and the normal interpretive code in pcre_exec() is run.
PCRE_ANCHORED
The PCRE_ANCHORED option limits pcre_exec() to matching at the first matching position. If a pattern was
compiled with PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made
unachored at matching time.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is
either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. These options override
the choice that was made or defaulted when the pattern was compiled.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the newline definition that was chosen or defaulted when the pattern was compiled.
For details, see the description of pcre_compile() above. During matching, the newline choice affects the
behaviour of the dot, circumflex, and dollar metacharacters. It may also alter the way the match position
is advanced after a match failure for an unanchored pattern.
When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a match attempt for an
unanchored pattern fails when the current position is at a CRLF sequence, and the pattern contains no
explicit matches for CR or LF characters, the match position is advanced by two characters instead of
one, in other words, to after the CRLF.
The above rule is a compromise that makes the most common cases work as expected. For example, if the
pattern is .+A (and the PCRE_DOTALL option is not set), it does not match the string "\r\nA" because,
after failing at the start, it skips both the CR and the LF before retrying. However, the pattern [\r\n]A
does match that string, because it contains an explicit CR or LF reference, and so advances only by one
character after the first failure.
An explicit match for CR of LF is either a literal appearance of one of those characters, or one of the
\r or \n escape sequences. Implicit matches such as [^X] do not count, nor does \s (which includes CR and
LF in the characters that it matches).
Notwithstanding the above, anomalous effects may still occur when CRLF is a valid newline sequence and
explicit \r or \n escapes appear in the pattern.
PCRE_NOTBOL
This option specifies that 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 PCRE_MULTILINE (at compile
time) causes circumflex never to match. This option affects only the behaviour of the circumflex
metacharacter. It does not affect \A.
PCRE_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 PCRE_MULTILINE (at compile time) causes dollar never to match. This option affects only the
behaviour of the dollar metacharacter. It does not affect \Z or \z.
PCRE_NOTEMPTY
An empty string is not considered to be a valid match if this option is set. 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 matches an empty string at the start of the
subject. With PCRE_NOTEMPTY set, this match is not valid, so PCRE searches further into the string for
occurrences of "a" or "b".
PCRE_NOTEMPTY_ATSTART
This is like PCRE_NOTEMPTY, except that an empty string match that is not at the start of the subject is
permitted. If the pattern is anchored, such a match can occur only if the pattern contains \K.
Perl has no direct equivalent of PCRE_NOTEMPTY or PCRE_NOTEMPTY_ATSTART, 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 behaviour after matching a null string by first trying the match again at the
same offset with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that fails, by advancing the
starting offset (see below) and trying an ordinary match again. There is some code that demonstrates how
to do this in the pcredemo sample program. In the most general case, you have to check to see if the
newline convention recognizes CRLF as a newline, and if so, and the current character is CR followed by
LF, advance the starting offset by two characters instead of one.
PCRE_NO_START_OPTIMIZE
There are a number of optimizations that pcre_exec() uses at the start of a match, in order to speed up
the process. For example, if it is known that an unanchored match must start with a specific character,
it searches the subject for that character, and fails immediately if it cannot find it, without actually
running the main matching function. This means that a special item such as (*COMMIT) at the start of a
pattern is not considered until after a suitable starting point for the match has been found. When
callouts or (*MARK) items are in use, these "start-up" optimizations can cause them to be skipped if the
pattern is never actually used. The start-up optimizations are in effect a pre-scan of the subject that
takes place before the pattern is run.
The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, possibly causing performance to
suffer, but ensuring that in cases where the result is "no match", the callouts do occur, and that items
such as (*COMMIT) and (*MARK) are considered at every possible starting position in the subject string.
If PCRE_NO_START_OPTIMIZE is set at compile time, it cannot be unset at matching time. The use of
PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching is always done using
interpretively.
Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching operation. Consider the pattern
(*COMMIT)ABC
When this is compiled, PCRE records the fact that a match must start with the character "A". Suppose the
subject string is "DEFABC". The start-up optimization scans along the subject, finds "A" and runs the
first match attempt from there. The (*COMMIT) item means that the pattern must match the current starting
position, which in this case, it does. However, if the same match is run with PCRE_NO_START_OPTIMIZE set,
the initial scan along the subject string does not happen. The first match attempt is run starting from
"D" and when this fails, (*COMMIT) prevents any further matches being tried, so the overall result is "no
match". If the pattern is studied, more start-up optimizations may be used. For example, a minimum length
for the subject may be recorded. Consider the pattern
(*MARK:A)(X|Y)
The minimum length for a match is one character. If the subject is "ABC", there will be attempts to match
"ABC", "BC", "C", and then finally an empty string. If the pattern is studied, the final attempt does
not take place, because PCRE knows that the subject is too short, and so the (*MARK) is never
encountered. In this case, studying the pattern does not affect the overall match result, which is still
"no match", but it does affect the auxiliary information that is returned.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set at compile time, the validity of the subject as a UTF-8 string is automatically
checked when pcre_exec() is subsequently called. The entire string is checked before any other
processing takes place. The value of startoffset is also checked to ensure that it points to the start of
a UTF-8 character. There is a discussion about the validity of UTF-8 strings in the pcreunicode page. If
an invalid sequence of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if
PCRE_PARTIAL_HARD is set and the problem is a truncated character at the end of the subject,
PCRE_ERROR_SHORTUTF8. In both cases, information about the precise nature of the error may also be
returned (see the descriptions of these errors in the section entitled Error return values from
pcre_exec() below). If startoffset contains a value that does not point to the start of a UTF-8
character (or to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.
If you already know that your subject is valid, and you want to skip these checks for performance
reasons, you can set the PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to do this
for the second and subsequent calls to pcre_exec() if you are making repeated calls to find all the
matches in a single subject string. However, you should be sure that the value of startoffset points to
the start of a character (or the end of the subject). When PCRE_NO_UTF8_CHECK is set, the effect of
passing an invalid string as a subject or an invalid value of startoffset is undefined. Your program may
crash.
PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT
These options turn on the partial matching feature. For backwards compatibility, PCRE_PARTIAL is a
synonym for PCRE_PARTIAL_SOFT. A partial match occurs if the end of the subject string is reached
successfully, but there are not enough subject characters to complete the match. If this happens when
PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, matching continues by testing any remaining
alternatives. Only if no complete match can be found is PCRE_ERROR_PARTIAL returned instead of
PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the caller is prepared to handle a
partial match, but only if no complete match can be found.
If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this case, if a partial match is found,
pcre_exec() immediately returns PCRE_ERROR_PARTIAL, without considering any other alternatives. In other
words, when PCRE_PARTIAL_HARD is set, a partial match is considered to be more important that an
alternative complete match.
In both cases, the portion of the string that was inspected when the partial match was found is set as
the first matching string. There is a more detailed discussion of partial and multi-segment matching,
with examples, in the pcrepartial documentation.
The string to be matched by pcre_exec()
The subject string is passed to pcre_exec() as a pointer in subject, a length in bytes in length, and a
starting byte offset in startoffset. If this is negative or greater than the length of the subject,
pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is zero, the search for a match starts
at the beginning of the subject, and this is by far the most common case. In UTF-8 mode, the byte offset
must point to the start of a UTF-8 character (or the end of the subject). Unlike the pattern string, the
subject may contain binary zero bytes.
A non-zero starting offset is useful when searching for another match in the same subject by calling
pcre_exec() again after a previous success. Setting startoffset differs from just passing over a
shortened string and setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the
subject is not a word boundary.) When applied to the string "Mississipi" the first call to pcre_exec()
finds the first occurrence. If pcre_exec() is called again with just the remainder of the subject, namely
"issipi", it does not match, because \B is always false at the start of the subject, which is deemed to
be a word boundary. However, if pcre_exec() is passed the entire string again, but with startoffset set
to 4, it finds the second occurrence of "iss" because it is able to look behind the starting point to
discover that it is preceded by a letter.
Finding all the matches in a subject is tricky when the pattern can match an empty string. It is possible
to emulate Perl's /g behaviour by first trying the match again at the same offset, with the
PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that fails, advancing the starting offset
and trying an ordinary match again. There is some code that demonstrates how to do this in the pcredemo
sample program. In the most general case, you have to check to see if the newline convention recognizes
CRLF as a newline, and if so, and the current character is CR followed by LF, advance the starting offset
by two characters instead of one.
If a non-zero starting offset is passed when the pattern is anchored, one attempt to match at the given
offset is made. This can only succeed if the pattern does not require the match to be at the start of the
subject.
How pcre_exec() returns captured substrings
In general, a pattern matches a certain portion of the subject, and in addition, further substrings from
the subject may be picked out by parts of the pattern. Following the usage in Jeffrey Friedl's book, this
is called "capturing" in what follows, and the phrase "capturing subpattern" is used for a fragment of a
pattern that picks out a substring. PCRE supports several other kinds of parenthesized subpattern that do
not cause substrings to be captured.
Captured substrings are returned to the caller via a vector of integers whose address is passed in
ovector. The number of elements in the vector is passed in ovecsize, which must be a non-negative number.
Note: this argument is NOT the size of ovector in bytes.
The first two-thirds of the vector is used to pass back captured substrings, each substring using a pair
of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching
capturing subpatterns, and is not available for passing back information. The number passed in ovecsize
should always be a multiple of three. If it is not, it is rounded down.
When a match is successful, information about captured substrings is returned in pairs of integers,
starting at the beginning of ovector, and continuing up to two-thirds of its length at the most. The
first element of each pair is set to the byte offset of the first character in a substring, and the
second is set to the byte offset of the first character after the end of a substring. Note: these values
are always byte offsets, even in UTF-8 mode. They are not character counts.
The first pair of integers, ovector[0] and ovector[1], identify the portion of the subject string matched
by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value
returned by pcre_exec() is one more than the highest numbered pair that has been set. For example, if
two substrings have been captured, the returned value is 3. If there are no capturing subpatterns, the
return value from a successful match is 1, indicating that just the first pair of offsets has been set.
If a capturing subpattern is matched repeatedly, it is the last portion of the string that it matched
that is returned.
If the vector is too small to hold all the captured substring offsets, it is used as far as possible (up
to two-thirds of its length), and the function returns a value of zero. If neither the actual string
matched nor any captured substrings are of interest, pcre_exec() may be called with ovector passed as
NULL and ovecsize as zero. However, if the pattern contains back references and the ovector is not big
enough to remember the related substrings, PCRE has to get additional memory for use during matching.
Thus it is usually advisable to supply an ovector of reasonable size.
There are some cases where zero is returned (indicating vector overflow) when in fact the vector is
exactly the right size for the final match. For example, consider the pattern
(a)(?:(b)c|bd)
If a vector of 6 elements (allowing for only 1 captured substring) is given with subject string "abd",
pcre_exec() will try to set the second captured string, thereby recording a vector overflow, before
failing to match "c" and backing up to try the second alternative. The zero return, however, does
correctly indicate that the maximum number of slots (namely 2) have been filled. In similar cases where
there is temporary overflow, but the final number of used slots is actually less than the maximum, a non-
zero value is returned.
The pcre_fullinfo() function can be used to find out how many capturing subpatterns there are in a
compiled pattern. The smallest size for ovector that will allow for n captured substrings, in addition to
the offsets of the substring matched by the whole pattern, is (n+1)*3.
It is possible for capturing subpattern number n+1 to match some part of the subject when subpattern n
has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the
return from the function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this happens, both
values in the offset pairs corresponding to unused subpatterns are set to -1.
Offset values that correspond to unused subpatterns at the end of the expression are also set to -1. For
example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
matched. The return from the function is 2, because the highest used capturing subpattern number is 1,
and the offsets for for the second and third capturing subpatterns (assuming the vector is large enough,
of course) are set to -1.
Note: Elements in the first two-thirds of ovector that do not correspond to capturing parentheses in the
pattern are never changed. That is, if a pattern contains n capturing parentheses, no more than
ovector[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in the first two-thirds) retain
whatever values they previously had.
Some convenience functions are provided for extracting the captured substrings as separate strings. These
are described below.
Error return values from pcre_exec()
If pcre_exec() fails, it returns a negative number. The following are defined in the header file:
PCRE_ERROR_NOMATCH (-1)
The subject string did not match the pattern.
PCRE_ERROR_NULL (-2)
Either code or subject was passed as NULL, or ovector was NULL and ovecsize was not zero.
PCRE_ERROR_BADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRE_ERROR_BADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is
passed a junk pointer and to detect when a pattern that was compiled in an environment of one endianness
is run in an environment with the other endianness. This is the error that PCRE gives when the magic
number is not present.
PCRE_ERROR_UNKNOWN_OPCODE (-5)
While running the pattern match, an unknown item was encountered in the compiled pattern. This error
could be caused by a bug in PCRE or by overwriting of the compiled pattern.
PCRE_ERROR_NOMEMORY (-6)
If a pattern contains back references, but the ovector that is passed to pcre_exec() is not big enough to
remember the referenced substrings, PCRE gets a block of memory at the start of matching to use for this
purpose. If the call via pcre_malloc() fails, this error is given. The memory is automatically freed at
the end of matching.
This error is also given if pcre_stack_malloc() fails in pcre_exec(). This can happen only when PCRE has
been compiled with --disable-stack-for-recursion.
PCRE_ERROR_NOSUBSTRING (-7)
This error is used by the pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list()
functions (see below). It is never returned by pcre_exec().
PCRE_ERROR_MATCHLIMIT (-8)
The backtracking limit, as specified by the match_limit field in a pcre_extra structure (or defaulted)
was reached. See the description above.
PCRE_ERROR_CALLOUT (-9)
This error is never generated by pcre_exec() itself. It is provided for use by callout functions that
want to yield a distinctive error code. See the pcrecallout documentation for details.
PCRE_ERROR_BADUTF8 (-10)
A string that contains an invalid UTF-8 byte sequence was passed as a subject, and the PCRE_NO_UTF8_CHECK
option was not set. If the size of the output vector (ovecsize) is at least 2, the byte offset to the
start of the the invalid UTF-8 character is placed in the first element, and a reason code is placed in
the second element. The reason codes are listed in the following section. For backward compatibility, if
PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 character at the end of the subject (reason
codes 1 to 5), PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
PCRE_ERROR_BADUTF8_OFFSET (-11)
The UTF-8 byte sequence that was passed as a subject was checked and found to be valid (the
PCRE_NO_UTF8_CHECK option was not set), but the value of startoffset did not point to the beginning of a
UTF-8 character or the end of the subject.
PCRE_ERROR_PARTIAL (-12)
The subject string did not match, but it did match partially. See the pcrepartial documentation for
details of partial matching.
PCRE_ERROR_BADPARTIAL (-13)
This code is no longer in use. It was formerly returned when the PCRE_PARTIAL option was used with a
compiled pattern containing items that were not supported for partial matching. From release 8.00
onwards, there are no restrictions on partial matching.
PCRE_ERROR_INTERNAL (-14)
An unexpected internal error has occurred. This error could be caused by a bug in PCRE or by overwriting
of the compiled pattern.
PCRE_ERROR_BADCOUNT (-15)
This error is given if the value of the ovecsize argument is negative.
PCRE_ERROR_RECURSIONLIMIT (-21)
The internal recursion limit, as specified by the match_limit_recursion field in a pcre_extra structure
(or defaulted) was reached. See the description above.
PCRE_ERROR_BADNEWLINE (-23)
An invalid combination of PCRE_NEWLINE_xxx options was given.
PCRE_ERROR_BADOFFSET (-24)
The value of startoffset was negative or greater than the length of the subject, that is, the value in
length.
PCRE_ERROR_SHORTUTF8 (-25)
This error is returned instead of PCRE_ERROR_BADUTF8 when the subject string ends with a truncated UTF-8
character and the PCRE_PARTIAL_HARD option is set. Information about the failure is returned as for
PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but this special error code for
PCRE_PARTIAL_HARD precedes the implementation of returned information; it is retained for backwards
compatibility.
PCRE_ERROR_RECURSELOOP (-26)
This error is returned when pcre_exec() detects a recursion loop within the pattern. Specifically, it
means that either the whole pattern or a subpattern has been called recursively for the second time at
the same position in the subject string. Some simple patterns that might do this are detected and faulted
at compile time, but more complicated cases, in particular mutual recursions between two different
subpatterns, cannot be detected until run time.
PCRE_ERROR_JIT_STACKLIMIT (-27)
This error is returned when a pattern that was successfully studied using a JIT compile option is being
matched, but the memory available for the just-in-time processing stack is not large enough. See the
pcrejit documentation for more details.
PCRE_ERROR_BADMODE (-28)
This error is given if a pattern that was compiled by the 8-bit library is passed to a 16-bit library
function, or vice versa.
PCRE_ERROR_BADENDIANNESS (-29)
This error is given if a pattern that was compiled and saved is reloaded on a host with different
endianness. The utility function pcre_pattern_to_host_byte_order() can be used to convert such a pattern
so that it runs on the new host.
Error numbers -16 to -20, -22, and -30 are not used by pcre_exec().
Reason codes for invalid UTF-8 strings
This section applies only to the 8-bit library. The corresponding information for the 16-bit library is
given in the pcre16 page.
When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORTUTF8, and the size of the output
vector (ovecsize) is at least 2, the offset of the start of the invalid UTF-8 character is placed in the
first output vector element (ovector[0]) and a reason code is placed in the second element (ovector[1]).
The reason codes are given names in the pcre.h header file:
PCRE_UTF8_ERR1
PCRE_UTF8_ERR2
PCRE_UTF8_ERR3
PCRE_UTF8_ERR4
PCRE_UTF8_ERR5
The string ends with a truncated UTF-8 character; the code specifies how many bytes are missing (1 to 5).
Although RFC 3629 restricts UTF-8 characters to be no longer than 4 bytes, the encoding scheme
(originally defined by RFC 2279) allows for up to 6 bytes, and this is checked first; hence the
possibility of 4 or 5 missing bytes.
PCRE_UTF8_ERR6
PCRE_UTF8_ERR7
PCRE_UTF8_ERR8
PCRE_UTF8_ERR9
PCRE_UTF8_ERR10
The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the character do not have the
binary value 0b10 (that is, either the most significant bit is 0, or the next bit is 1).
PCRE_UTF8_ERR11
PCRE_UTF8_ERR12
A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long; these code points are
excluded by RFC 3629.
PCRE_UTF8_ERR13
A 4-byte character has a value greater than 0x10fff; these code points are excluded by RFC 3629.
PCRE_UTF8_ERR14
A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of code points are reserved by
RFC 3629 for use with UTF-16, and so are excluded from UTF-8.
PCRE_UTF8_ERR15
PCRE_UTF8_ERR16
PCRE_UTF8_ERR17
PCRE_UTF8_ERR18
PCRE_UTF8_ERR19
A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a value that can be
represented by fewer bytes, which is invalid. For example, the two bytes 0xc0, 0xae give the value 0x2e,
whose correct coding uses just one byte.
PCRE_UTF8_ERR20
The two most significant bits of the first byte of a character have the binary value 0b10 (that is, the
most significant bit is 1 and the second is 0). Such a byte can only validly occur as the second or
subsequent byte of a multi-byte character.
PCRE_UTF8_ERR21
The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8
string.
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the offsets returned by pcre_exec() in ovector. For
convenience, the functions pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() are
provided for extracting captured substrings as new, separate, zero-terminated strings. These functions
identify substrings by number. The next section describes functions for extracting named substrings.
A substring that contains a binary zero is correctly extracted and has a further zero added on the end,
but the result is not, of course, a C string. However, you can process such a string by referring to the
length that is returned by pcre_copy_substring() and pcre_get_substring(). Unfortunately, the interface
to pcre_get_substring_list() is not adequate for handling strings containing binary zeros, because the
end of the final string is not independently indicated.
The first three arguments are the same for all three of these functions: subject is the subject string
that has just been successfully matched, ovector is a pointer to the vector of integer offsets that was
passed to pcre_exec(), and stringcount is the number of substrings that were captured by the match,
including the substring that matched the entire regular expression. This is the value returned by
pcre_exec() if it is greater than zero. If pcre_exec() returned zero, indicating that it ran out of space
in ovector, the value passed as stringcount should be the number of elements in the vector divided by
three.
The functions pcre_copy_substring() and pcre_get_substring() extract a single substring, whose number is
given as stringnumber. A value of zero extracts the substring that matched the entire pattern, whereas
higher values extract the captured substrings. For pcre_copy_substring(), the string is placed in buffer,
whose length is given by buffersize, while for pcre_get_substring() a new block of memory is obtained via
pcre_malloc, and its address is returned via stringptr. The yield of the function is the length of the
string, not including the terminating zero, or one of these error codes:
PCRE_ERROR_NOMEMORY (-6)
The buffer was too small for pcre_copy_substring(), or the attempt to get memory failed for
pcre_get_substring().
PCRE_ERROR_NOSUBSTRING (-7)
There is no substring whose number is stringnumber.
The pcre_get_substring_list() function extracts all available substrings and builds a list of pointers to
them. All this is done in a single block of memory that is obtained via pcre_malloc. The address of the
memory block is returned via listptr, which is also the start of the list of string pointers. The end of
the list is marked by a NULL pointer. The yield of the function is zero if all went well, or the error
code
PCRE_ERROR_NOMEMORY (-6)
if the attempt to get the memory block failed.
When any of these functions encounter a substring that is unset, which can happen when capturing
subpattern number n+1 matches some part of the subject, but subpattern n has not been used at all, they
return an empty string. This can be distinguished from a genuine zero-length substring by inspecting the
appropriate offset in ovector, which is negative for unset substrings.
The two convenience functions pcre_free_substring() and pcre_free_substring_list() can be used to free
the memory returned by a previous call of pcre_get_substring() or pcre_get_substring_list(),
respectively. They do nothing more than call the function pointed to by pcre_free, which of course could
be called directly from a C program. However, PCRE is used in some situations where it is linked via a
special interface to another programming language that cannot use pcre_free directly; it is for these
cases that the functions are provided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find associated number. For example, for this pattern
(a+)b(?<xxx>\d+)...
the number of the subpattern called "xxx" is 2. If the name is known to be unique (PCRE_DUPNAMES was not
set), you can find the number from the name by calling pcre_get_stringnumber(). The first argument is the
compiled pattern, and the second is the name. The yield of the function is the subpattern number, or
PCRE_ERROR_NOSUBSTRING (-7) if there is no subpattern of that name.
Given the number, you can extract the substring directly, or use one of the functions described in the
previous section. For convenience, there are also two functions that do the whole job.
Most of the arguments of pcre_copy_named_substring() and pcre_get_named_substring() are the same as those
for the similarly named functions that extract by number. As these are described in the previous section,
they are not re-described here. There are just two differences:
First, instead of a substring number, a substring name is given. Second, there is an extra argument,
given at the start, which is a pointer to the compiled pattern. This is needed in order to gain access to
the name-to-number translation table.
These functions call pcre_get_stringnumber(), and if it succeeds, they then call pcre_copy_substring() or
pcre_get_substring(), as appropriate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
behaviour may not be what you want (see the next section).
Warning: If the pattern uses the (?| feature to set up multiple subpatterns with the same number, as
described in the section on duplicate subpattern numbers in the pcrepattern page, you cannot use names to
distinguish the different subpatterns, because names are not included in the compiled code. The matching
process uses only numbers. For this reason, the use of different names for subpatterns of the same number
causes an error at compile time.
DUPLICATE SUBPATTERN NAMES
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
When a pattern is compiled with the PCRE_DUPNAMES option, names for subpatterns are not required to be
unique. (Duplicate names are always allowed for subpatterns with the same number, created by using the
(?| feature. Indeed, if such subpatterns are named, they are required to use the same names.)
Normally, patterns with duplicate names are such that in any one match, only one of the named subpatterns
participates. An example is shown in the pcrepattern documentation.
When duplicates are present, pcre_copy_named_substring() and pcre_get_named_substring() return the first
substring corresponding to the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is
returned; no data is returned. The pcre_get_stringnumber() function returns one of the numbers that are
associated with the name, but it is not defined which it is.
If you want to get full details of all captured substrings for a given name, you must use the
pcre_get_stringtable_entries() function. The first argument is the compiled pattern, and the second is
the name. The third and fourth are pointers to variables which are updated by the function. After it has
run, they point to the first and last entries in the name-to-number table for the given name. The
function itself returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if there are none. The
format of the table is described above in the section entitled Information about a pattern above. Given
all the relevant entries for the name, you can extract each of their numbers, and hence the captured
data, if any.
FINDING ALL POSSIBLE MATCHES
The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first
match, starting at a given point in the subject. If you want to find all possible matches, or the longest
possible match, consider using the alternative matching function (see below) instead. If you cannot use
the alternative function, but still need to find all possible matches, you can kludge it up by making use
of the callout facility, which is described in the pcrecallout documentation.
What you have to do is to insert a callout right at the end of the pattern. When your callout function
is called, extract and save the current matched substring. Then return 1, which forces pcre_exec() to
backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre_exec() will yield
PCRE_ERROR_NOMATCH.
OBTAINING AN ESTIMATE OF STACK USAGE
Matching certain patterns using pcre_exec() can use a lot of process stack, which in certain environments
can be rather limited in size. Some users find it helpful to have an estimate of the amount of stack that
is used by pcre_exec(), to help them set recursion limits, as described in the pcrestack documentation.
The estimate that is output by pcretest when called with the -m and -C options is obtained by calling
pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its first five arguments.
Normally, if its first argument is NULL, pcre_exec() immediately returns the negative error code
PCRE_ERROR_NULL, but with this special combination of arguments, it returns instead a negative number
whose absolute value is the approximate stack frame size in bytes. (A negative number is used so that it
is clear that no match has happened.) The value is approximate because in some cases, recursive calls to
pcre_exec() occur when there are one or two additional variables on the stack.
If PCRE has been compiled to use the heap instead of the stack for recursion, the value returned is the
size of each block that is obtained from the heap.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
The function pcre_dfa_exec() is called to match a subject string against a compiled pattern, using a
matching algorithm that scans the subject string just once, and does not backtrack. This has different
characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE
patterns are not supported. Nevertheless, there are times when this kind of matching can be useful. For a
discussion of the two matching algorithms, and a list of features that pcre_dfa_exec() does not support,
see the pcrematching documentation.
The arguments for the pcre_dfa_exec() function are the same as for pcre_exec(), plus two extras. The
ovector argument is used in a different way, and this is described below. The other common arguments are
used in the same way as for pcre_exec(), so their description is not repeated here.
The two additional arguments provide workspace for the function. The workspace vector should contain at
least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More
workspace will be needed for patterns and subjects where there are a lot of potential matches.
Here is an example of a simple call to pcre_dfa_exec():
int rc;
int ovector[10];
int wspace[20];
rc = pcre_dfa_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
10, /* number of elements (NOT size in bytes) */
wspace, /* working space vector */
20); /* number of elements (NOT size in bytes) */
Option bits for pcre_dfa_exec()
The unused bits of the options argument for pcre_dfa_exec() must be zero. The only bits that may be set
are PCRE_ANCHORED, PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD,
PCRE_PARTIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last four of these are exactly
the same as for pcre_exec(), so their description is not repeated here.
PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT
These have the same general effect as they do for pcre_exec(), but the details are slightly different.
When PCRE_PARTIAL_HARD is set for pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the
subject is reached and there is still at least one matching possibility that requires additional
characters. This happens even if some complete matches have also been found. When PCRE_PARTIAL_SOFT is
set, the return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the subject is
reached, there have been no complete matches, but there is still at least one matching possibility. The
portion of the string that was inspected when the longest partial match was found is set as the first
matching string in both cases. There is a more detailed discussion of partial and multi-segment
matching, with examples, in the pcrepartial documentation.
PCRE_DFA_SHORTEST
Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to stop as soon as it has found one
match. Because of the way the alternative algorithm works, this is necessarily the shortest possible
match at the first possible matching point in the subject string.
PCRE_DFA_RESTART
When pcre_dfa_exec() returns a partial match, it is possible to call it again, with additional subject
characters, and have it continue with the same match. The PCRE_DFA_RESTART option requests this action;
when it is set, the workspace and wscount options must reference the same vector as before because data
about the match so far is left in them after a partial match. There is more discussion of this facility
in the pcrepartial documentation.
Successful returns from pcre_dfa_exec()
When pcre_dfa_exec() succeeds, it may have matched more than one substring in the subject. Note, however,
that all the matches from one run of the function start at the same point in the subject. The shorter
matches are all initial substrings of the longer matches. For example, if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further> no more
the three matched strings are
<something>
<something> <something else>
<something> <something else> <something further>
On success, the yield of the function is a number greater than zero, which is the number of matched
substrings. The substrings themselves are returned in ovector. Each string uses two elements; the first
is the offset to the start, and the second is the offset to the end. In fact, all the strings have the
same start offset. (Space could have been saved by giving this only once, but it was decided to retain
some compatibility with the way pcre_exec() returns data, even though the meaning of the strings is
different.)
The strings are returned in reverse order of length; that is, the longest matching string is given first.
If there were too many matches to fit into ovector, the yield of the function is zero, and the vector is
filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() can use the entire ovector for
returning matched strings.
Error returns from pcre_dfa_exec()
The pcre_dfa_exec() function returns a negative number when it fails. Many of the errors are the same as
for pcre_exec(), and these are described above. There are in addition the following errors that are
specific to pcre_dfa_exec():
PCRE_ERROR_DFA_UITEM (-16)
This return is given if pcre_dfa_exec() encounters an item in the pattern that it does not support, for
instance, the use of \C or a back reference.
PCRE_ERROR_DFA_UCOND (-17)
This return is given if pcre_dfa_exec() encounters a condition item that uses a back reference for the
condition, or a test for recursion in a specific group. These are not supported.
PCRE_ERROR_DFA_UMLIMIT (-18)
This return is given if pcre_dfa_exec() is called with an extra block that contains a setting of the
match_limit or match_limit_recursion fields. This is not supported (these fields are meaningless for DFA
matching).
PCRE_ERROR_DFA_WSSIZE (-19)
This return is given if pcre_dfa_exec() runs out of space in the workspace vector.
PCRE_ERROR_DFA_RECURSE (-20)
When a recursive subpattern is processed, the matching function calls itself recursively, using private
vectors for ovector and workspace. This error is given if the output vector is not large enough. This
should be extremely rare, as a vector of size 1000 is used.
PCRE_ERROR_DFA_BADRESTART (-30)
When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some plausibility checks are made on the
contents of the workspace, which should contain data about the previous partial match. If any of these
checks fail, this error is given.
SEE ALSO
pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepartial(3), pcreposix(3),
pcreprecompile(3), pcresample(3), pcrestack(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 17 June 2012
Copyright (c) 1997-2012 University of Cambridge.