PCRE(3)

NAME
PCRE - Perl-compatible regular expressions

SYNOPSIS OF PCRE API

#include <pcre.h>

pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);

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_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_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);

const unsigned char *pcre_maketables(void);

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);

int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

int pcre_config(int what, void *where);

char *pcre_version(void);

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

int (*pcre_callout)(pcre_callout_block *);

PCRE API

PCRE has its own native API, which is described in this document. There is also a set of wrapper functions that correspond to the POSIX regular expression API. These are described in the pcreposix documentation.

The native API function prototypes are defined in the header file pcre.h, and on Unix systems the library itself is called libpcre.a, so can be accessed by adding -lpcre to the command for linking an application which calls it. 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.

The functions pcre_compile(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions. A sample program that demonstrates the simplest way of using them is given in the file pcredemo.c. The pcresample documentation describes how to run it.

There are convenience functions for extracting captured substrings from a matched subject string. They are:

  pcre_copy_substring()
  pcre_copy_named_substring()
  pcre_get_substring()
  pcre_get_named_substring()
  pcre_get_substring_list()

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 (optionally) to build a set of character tables in the current locale for passing to pcre_compile().

The function pcre_fullinfo() is used to find out information about a compiled pattern; pcre_info() is an obsolete version which returns only some of the available information, but is retained for backwards compatibility. The function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release.

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 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.

MULTITHREADING

The PCRE functions can be used in multi-threading applications, with the proviso that the memory management functions pointed to by pcre_malloc and pcre_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.

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 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.

  PCRE_CONFIG_NEWLINE

The output is an integer that is set to the value of the code that is used for the newline character. It is either linefeed (10) or carriage return (13), and should normally be the standard character for your operating system.

  PCRE_CONFIG_LINK_SIZE

The output is an integer that contains the number of bytes used for internal linkage in compiled regular expressions. The value is 2, 3, or 4. Larger values allow larger regular expressions to be compiled, at the expense of slower matching. 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.

  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 an 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.

COMPILING A PATTERN

pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

The function pcre_compile() is called to compile a pattern into an internal form. The pattern is a C string terminated by a binary zero, and is passed in the argument pattern. 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 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 contains a copy of the tableptr argument, which is an address (see below).

The options argument contains independent bits that affect the compilation. It should be zero if no options are required. Some of the options, in particular, those that are compatible with Perl, can also be set and unset from within the pattern (see the detailed description of regular expressions in the pcrepattern documentation). For these options, the contents of the options argument specifies their initial settings at the start of compilation and execution. The PCRE_ANCHORED option 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. The offset from the start of the pattern to the character where 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.

If the final argument, tableptr, is NULL, PCRE uses a default set of character tables which are built when it is compiled, using the default C locale. Otherwise, tableptr must be the result of a call to pcre_maketables(). 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 option bits are defined:

  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 which 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_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.

  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 the final character if it is a newline (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 metacharater in the pattern matches all characters, including newlines. Without it, newlines are excluded. 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 a newline character, independent of the setting of this option.

  PCRE_EXTENDED

If this bit is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class. Whitespace does not include the VT character (code 11). In addition, characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting.

This option makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.

  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. 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_MULTILINE

By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several 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 any newline 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 "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect.

  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.

  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 character strings. However, it is available only if PCRE has been built to include UTF-8 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 section on UTF-8 support in the main pcre page.

  PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is automatically checked. If an invalid UTF-8 sequence of bytes 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 there is a similar option for suppressing the checking of subject strings passed to pcre_exec().

STUDYING A PATTERN

pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);

When a 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 studing 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 a pcre_study() can be passed directly to pcre_exec(). However, the pcre_extra block also contains other fields that can be set by the caller before the block is passed; these are described below. If studying the pattern does not produce any additional information, pcre_study() returns NULL. In that circumstance, if the calling program wants to pass some of the other fields to pcre_exec(), it must set up its own pcre_extra block.

The second argument contains option bits. At present, no options are defined for pcre_study(), and this argument should always be zero.

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 points to a textual error message. You should therefore test the error pointer for NULL after calling pcre_study(), to be sure that it has run successfully.

This is a typical call to pcre_study():

  pcre_extra *pe;
  pe = pcre_study(
    re, /* result of pcre_compile() */
    0, /* no options exist */
    &error); /* set to NULL or points to a message */

At present, studying a pattern is useful only for non-anchored patterns that do not have a single fixed starting character. A bitmap of possible starting characters is created.

LOCALE SUPPORT

PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables. When running in UTF-8 mode, this applies only to characters with codes less than 256. The library contains a default set of tables that is created in the default C locale when PCRE is compiled. This is used when the final argument of pcre_compile() is NULL, and is sufficient for many applications.

An alternative set of tables can, however, be supplied. Such 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() as often as necessary. For example, to build and use tables that are appropriate for the French locale (where accented characters with codes greater than 128 are treated as letters), the following code could be used:

  setlocale(LC_CTYPE, "fr");
  tables = pcre_maketables();
  re = pcre_compile(..., tables);

The tables are built in memory that is obtained via pcre_malloc. 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 pcre_exec(). Thus, 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 the caller's responsibility to ensure that the memory containing the tables remains available for as long as it is needed.

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 obsolete pcre_info() function, which is nevertheless retained for backwards compability (and is documented below).

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_BADOPTION the value of what was invalid

Here is a typical call of pcre_fullinfo(), to obtain the length of the compiled pattern:

  int rc;
  unsigned long int length;
  rc = pcre_fullinfo(
    re, /* result of pcre_compile() */
    pe, /* 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_FIRSTBYTE

Return information about the first byte of any matched string, for a non-anchored pattern. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name is still recognized for backwards compatibility.)

If there is a fixed first byte, e.g. from a pattern such as (cat|cow|coyote), it is returned in the integer pointed to by where. Otherwise, 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 bytes for the first byte 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_LASTLITERAL

Return the value of the rightmost literal byte that must exist in any matched string, other than at its start, if such a byte has been recorded. The fourth argument should point to an int variable. If there is no such byte, -1 is returned. For anchored patterns, a last literal byte 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_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 a number. A caller that wants to extract data from a named subpattern must convert the name to a number in order to access the correct pointers in the output vector (described with pcre_exec() below). In order to do this, it must first use these three values to obtain the name-to-number mapping table for the pattern.

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 (a pointer to char). The first two bytes of each entry are the number of the capturing parenthesis, most significant byte first. The rest of the entry is the corresponding name, zero terminated. The names are in alphabetical order. For example, consider the following pattern (assume PCRE_EXTENDED is set, so white space - including newlines - is ignored):

  (?P<date> (?P<year>(\d\d)?\d\d) -
  (?P<month>\d\d) - (?P<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 hex, 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, remember that the length of each entry may be different for each compiled pattern.

  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 within the pattern itself.

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, that is, the value that was passed as the argument to pcre_malloc() when PCRE was getting memory in which to place the compiled data. The fourth argument should point to a size_t variable.

  PCRE_INFO_STUDYSIZE

Returns the size of the data block pointed to by the study_data field in a pcre_extra block. That is, it is the value that was passed to pcre_malloc() when PCRE was getting memory into which to place the data created by pcre_study(). The fourth argument should point to a size_t variable.

OBSOLETE INFO FUNCTION

int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

The pcre_info() function is now obsolete because its interface is too restrictive to return all the available data about a compiled pattern. New programs should use pcre_fullinfo() instead. The yield of pcre_info() is the number of capturing subpatterns, or one of the following negative numbers:

  PCRE_ERROR_NULL the argument code was NULL
  PCRE_ERROR_BADMAGIC the "magic number" was not found

If the optptr argument is not NULL, a copy of the options with which the pattern was compiled is placed in the integer it points to (see PCRE_INFO_OPTIONS above).

If the pattern is not anchored and the firstcharptr argument is not NULL, it is used to pass back information about the first character of any matched string (see PCRE_INFO_FIRSTBYTE above).

MATCHING A PATTERN

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 pre-compiled pattern, which is passed in the code argument. If the pattern has been studied, the result of the study should be passed in the extra argument.

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 for substring information */
    30); /* number of elements in the vector */

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 fields in the block are as follows:

  unsigned long int flags;
  void *study_data;
  unsigned long int match_limit;
  void *callout_data;

The flags field is a bitmap that specifies which of the other fields are set. The flag bits are:

  PCRE_EXTRA_STUDY_DATA
  PCRE_EXTRA_MATCH_LIMIT
  PCRE_EXTRA_CALLOUT_DATA

Other flag bits should be set to zero. The study_data field is set in the pcre_extra block that is returned by pcre_study(), together with the appropriate flag bit. You should not set this yourself, but you can add to the block by setting the other fields.

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 the use of nested unlimited repeats. Internally, PCRE uses a function called match() which it calls repeatedly (sometimes recursively). The limit is imposed on the number of times this function is called during a match, which has the effect of limiting the amount of recursion and backtracking that can take place. For patterns that are not anchored, the count starts from zero for each position in the subject string.

The default limit for the library can be set when PCRE is built; the default default is 10 million, which handles all but the most extreme cases. You can reduce the default by suppling pcre_exec() with a pcre_extra block in which match_limit is set to a smaller value, and PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

The pcre_callout field is used in conjunction with the "callout" feature, which is described in the pcrecallout documentation.

The PCRE_ANCHORED option can be passed in the options argument, whose unused bits must be zero. This limits pcre_exec() to matching at the first matching position. However, 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.

When PCRE_UTF8 was set at compile time, the validity of the subject as a UTF-8 string is automatically checked. If an invalid UTF-8 sequence of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If you already know that your subject is valid, and you want to skip this check for performance reasons, you can set the PCRE_NO_UTF8_CHECK option when calling pcre_exec(). When this option is set, the effect of passing an invalid UTF-8 string as a subject is undefined. It may cause your program to crash.

There are also three further options that can be set only at matching time:

  PCRE_NOTBOL

The first character of the 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.

  PCRE_NOTEOL

The end of the 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.

  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 the 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".

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

The subject string is passed to pcre_exec() as a pointer in subject, a length in length, and a starting offset in startoffset. Unlike the pattern string, the subject may contain binary zero bytes. 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.

If the pattern was compiled with the PCRE_UTF8 option, the subject must be a sequence of bytes that is a valid UTF-8 string. If an invalid UTF-8 string is passed, PCRE's behaviour is not defined.

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.

If a non-zero starting offset is passed when the pattern is anchored, one attempt to match at the given offset is tried. This can only succeed if the pattern does not require the match to be at the start of the subject.

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 integer offsets whose address is passed in ovector. The number of elements in the vector is passed in ovecsize. 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 length passed in ovecsize should always be a multiple of three. If it is not, it is rounded down.

When a match has been 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 a pair is set to the offset of the first character in a substring, and the second is set to the offset of the first character after the end of a substring. The first pair, 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 the number of pairs that have been set. 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.

Some convenience functions are provided for extracting the captured substrings as separate strings. These are described in the following section.

It is possible for an 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) subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset values corresponding to the unused subpattern are set to -1.

If a capturing subpattern is matched repeatedly, it is the last portion of the string that it matched that gets returned.

If the vector is too small to hold all the captured substrings, it is used as far as possible (up to two-thirds of its length), and the function returns a value of zero. In particular, if the substring offsets are not 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 isn't 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.

Note that pcre_info() 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.

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. This is the error it gives when the magic number isn't present.

  PCRE_ERROR_UNKNOWN_NODE (-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 freed at the end of matching.

  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 recursion and 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.

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.

The first three arguments are the same for all three of these functions: subject is the subject string which 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 size of 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, while 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

  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 which 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

  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 which cannot use pcre_free directly; it is for these cases that the functions are provided.

EXTRACTING CAPTURED SUBSTRINGS BY 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_stringnumber(const pcre *code,
const char *name);

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. This can be done by calling pcre_get_stringnumber(). The first argument is the compiled pattern, and the second is the name. For example, for this pattern

  ab(?<xxx>\d+)...

the number of the subpattern called "xxx" is 1. Given the number, you can then 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 functions that extract by number, and so 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.

Last updated: 20 August 2003
Copyright (c) 1997-2003 University of Cambridge.