Manpages

PCRE

NAME

SYNOPSIS

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_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);

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);

char *pcre_version(void);

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

DESCRIPTION

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.

The functions pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() are convenience functions for extracting captured substrings from a matched subject string; 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.

MULTI-THREADING

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.

COMPILING A PATTERN

The size of a compiled pattern is roughly proportional to the length of the pattern string, except that each character class (other than those containing just a single character, negated or not) requires 33 bytes, and repeat quantifiers with a minimum greater than one or a bounded maximum cause the relevant portions of the compiled pattern to be replicated.

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 below). 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.

The following option bits are defined in the 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 start of 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.

  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.

  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. 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, and 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 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. 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_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 just byte 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. Support for UTF-8 is new, experimental, and incomplete. Details of exactly what it entails are given below.

STUDYING A PATTERN

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 to 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.

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

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

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, while 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

The possible values for the third argument are defined in pcre.h, and are as follows:

  PCRE_INFO_OPTIONS

Return a copy of the options with which the pattern was compiled. The fourth argument should point to au 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, and with the PCRE_ANCHORED bit forcibly set if the form of the pattern implies that it can match only at the start of a subject string.

  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_CAPTURECOUNT

Return the number of capturing subpatterns in the pattern. The fourth argument should point to an int variable.

  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_FIRSTCHAR

Return information about the first character of any matched string, for a non-anchored pattern. If there is a fixed first character, 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 "\n" 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 characters for the first character 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

For a non-anchored pattern, return the value of the rightmost literal character which must exist in any matched string, other than at its start. The fourth argument should point to an int variable. If there is no such character, or if the pattern is anchored, -1 is returned. For example, for the pattern /a\d+z\d+/ the returned value is 'z'.

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_FIRSTCHAR above).

MATCHING A PATTERN

The PCRE_ANCHORED option can be passed in the options argument, whose unused bits must be zero. 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.

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 as a pointer in subject, a length in length, and a starting offset in startoffset. Unlike the pattern string, it may contain binary zero characters. 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.

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.

EXTRACTING CAPTURED SUBSTRINGS

The first three arguments are the same for all three 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.

LIMITATIONS

The maximum length of a subject string is the largest positive number that an integer variable can hold. However, PCRE uses recursion to handle subpatterns and indefinite repetition. This means that the available stack space may limit the size of a subject string that can be processed by certain patterns.

AUTHOR

Last updated: 28 August 2000,

  the 250th anniversary of the death of J.S. Bach.
Copyright (c) 1997-2000 University of Cambridge.

Index

NAME

SYNOPSIS

DESCRIPTION

MULTI-THREADING

COMPILING A PATTERN

STUDYING A PATTERN

LOCALE SUPPORT

INFORMATION ABOUT A PATTERN

MATCHING A PATTERN

EXTRACTING CAPTURED SUBSTRINGS

LIMITATIONS

AUTHOR