pcreapi specification

# pcreapi man page

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- PCRE NATIVE API
- PCRE API OVERVIEW
- MULTITHREADING
- SAVING PRECOMPILED PATTERNS FOR LATER USE
- CHECKING BUILD-TIME OPTIONS
- COMPILING A PATTERN
- STUDYING A PATTERN
- LOCALE SUPPORT
- INFORMATION ABOUT A PATTERN
- OBSOLETE INFO FUNCTION
- MATCHING A PATTERN
- EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
- EXTRACTING CAPTURED SUBSTRINGS BY NAME

PCRE NATIVE 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 *);

void *(*pcre_stack_malloc)(size_t);

void (*pcre_stack_free)(void *);

int (*pcre_callout)(pcre_callout_block *);

PCRE API OVERVIEW

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. 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.

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 provided in the file called pcredemo.c in the source distribution. The pcresample documentation describes how to run it.

In addition to the main compiling and matching functions, 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_get_stringnumber()

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() or pcre_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; pcre_info() is an obsolete version that 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 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. This 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.

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, 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.

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.

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_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_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.

      PCRE_CONFIG_STACKRECURSE

The output is an integer that is set to one if internal recursion 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);

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 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 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 independent bits 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, can also be set and unset from within the pattern (see the detailed description 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 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_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. When running in UTF-8 mode, case support for high-valued characters is available only when PCRE is built with Unicode character property 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 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 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 when PCRE is 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 this option can also be passed to pcre_exec(), to suppress the UTF-8 validity checking of subject strings.
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(). 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 additional information, pcre_study() returns NULL. In that circumstance, if the calling program wants to pass any of the other fields to pcre_exec(), it must set up its own pcre_extra block.

The second argument of pcre_study() contains option bits. At present, no options are defined, 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 bytes is created.
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. Higher-valued codes never match escapes such as \w or \d, but can be tested with \p if PCRE is built with Unicode character property support.)

An internal set of tables is created in the default C locale when PCRE is built. 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. 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);

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 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

The "magic number" is placed at the start of each compiled pattern as an simple check against passing an arbitrary memory pointer. 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_DEFAULTTABLES

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 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, for example, 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 numbers. A convenience function called pcre_get_named_substring() is provided for extracting an individual captured substring 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 (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 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 each entry is likely to 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

Return 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 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.

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 in the vector (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 fields in a pcre_extra block are as follows:

      unsigned long int flags;
      void *study_data;
      unsigned long int match_limit;
      void *callout_data;
      const unsigned char *tables;

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
      PCRE_EXTRA_TABLES

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 may add to the block by setting the 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 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 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.

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_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.

      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_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 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 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.c sample program.

      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 value of startoffset is also checked to ensure that it points to the start of a UTF-8 character. If an invalid UTF-8 sequence of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset contains an invalid value, 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 UTF-8 character. When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8 string as a subject, or a value of startoffset that does not point to the start of a UTF-8 character, is undefined. Your program may crash.

      PCRE_PARTIAL

This option turns on the partial matching feature. If the subject string fails to match the pattern, but at some point during the matching process the end of the subject was reached (that is, the subject partially matches the pattern and the failure to match occurred only because there were not enough subject characters), pcre_exec() returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is used, there are restrictions on what may appear in the pattern. These are discussed 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 length, and a starting byte offset in startoffset. In UTF-8 mode, the byte offset must point to the start of a UTF-8 character. 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.

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 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 integer offsets 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 length 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 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 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. 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 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.

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.

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_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 automatically 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.

      PCRE_ERROR_BADUTF8_OFFSET (-11)

The UTF-8 byte sequence that was passed as a subject was valid, but the value of startoffset did not point to the beginning of a UTF-8 character.

      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_BAD_PARTIAL (-13)

The PCRE_PARTIAL option was used with a compiled pattern containing items that are not supported for partial matching. See the pcrepartial documentation for details of 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.
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 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

      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

      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_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. 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.

Last updated: 09 September 2004
Copyright © 1997-2004 University of Cambridge.

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