PCRE NATIVE API

#include <pcre.h>

PCRE API OVERVIEW

PCRE has its own native API, which is described in this document. There are also some wrapper functions that correspond to the POSIX regular expression API. These are described in the pcreposix(3) documentation. Both of these APIs define a set of C function calls. A C++ wrapper is distributed with PCRE. It is documented in the pcrecpp(3) page.

The native API C 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.

In a Windows environment, if you want to statically link an application program against a non-dll pcre.a file, you must define PCRE_STATIC before including pcre.h or pcrecpp.h, because otherwise the pcre_malloc() and pcre_free() exported functions will be declared __declspec(dllimport), with unwanted results.

The functions pcre_compile(), pcre_compile2(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the simplest way of using them is provided in the file called pcredemo.c in the PCRE source distribution. A listing of this program is given in the pcredemo(3) documentation, and the pcresample(3) documentation describes how to compile and run it.

A second matching function, pcre_dfa_exec(), which is not Perl-compatible, is also provided. This uses a different algorithm for the matching. The alternative algorithm finds all possible matches (at a given point in the subject), and scans the subject just once (unless there are lookbehind assertions). However, this algorithm does not return captured substrings. A description of the two matching algorithms and their advantages and disadvantages is given in the pcrematching(3) documentation.

In addition to the main compiling and matching functions, there are convenience functions for extracting captured substrings from a subject string that is matched by pcre_exec(). They are:

 pcre_copy_substring()
 pcre_copy_named_substring()
 pcre_get_substring()
 pcre_get_named_substring()
 pcre_get_substring_list()
 pcre_get_stringnumber()
 pcre_get_stringtable_entries()

pcre_free_substring() and pcre_free_substring_list() are also provided, to free the memory used for extracted strings.

The function pcre_maketables() is used to build a set of character tables in the current locale for passing to pcre_compile(), pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is provided for specialist use. Most commonly, no special tables are passed, in which case internal tables that are generated when PCRE is built are used.

The function pcre_fullinfo() is used to find out information about a compiled pattern; 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 function pcre_refcount() maintains a reference count in a data block containing a compiled pattern. This is provided for the benefit of object-oriented applications.

The global variables pcre_malloc and pcre_free initially contain the entry points of the standard malloc() and free() functions, respectively. PCRE calls the memory management functions via these variables, so a calling program can replace them if it wishes to intercept the calls. This should be done before calling any PCRE functions.

The global variables pcre_stack_malloc and pcre_stack_free are also indirections to memory management functions. These special functions are used only when PCRE is compiled to use the heap for remembering data, instead of recursive function calls, when running the pcre_exec() function. See the pcrebuild(3) documentation for details of how to do this. It is a non-standard way of building PCRE, for use in environments that have limited stacks. Because of the greater use of memory management, it runs more slowly. Separate functions are provided so that special-purpose external code can be used for this case. When used, these functions are always called in a stack-like manner (last obtained, first freed), and always for memory blocks of the same size. There is a discussion about PCRE's stack usage in the pcrestack(3) documentation.

The global variable pcre_callout initially contains NULL. It can be set by the caller to a "callout" function, which PCRE will then call at specified points during a matching operation. Details are given in the pcrecallout(3) documentation.

NEWLINES

PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) character, the two-character sequence CRLF, any of the three preceding, or any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029).

Each of the first three conventions is used by at least one operating system as its standard newline sequence. When PCRE is built, a default can be specified. The default default is LF, which is the Unix standard. When PCRE is run, the default can be overridden, either when a pattern is compiled, or when it is matched.

At compile time, the newline convention can be specified by the options argument of pcre_compile(), or it can be specified by special text at the start of the pattern itself; this overrides any other settings. See the pcrepattern(3) page for details of the special character sequences.

In the PCRE documentation the word "newline" is used to mean "the character or pair of characters that indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending sequence, the match position advancement for a non-anchored pattern. There is more detail about this in the section on pcre_exec() options below.

The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor does it affect what \R matches, which is controlled in a similar way, but by separate options.

MULTITHREADING

The PCRE functions can be used in multi-threading applications, with the proviso that the memory management functions pointed to by pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the callout function pointed to by pcre_callout, are shared by all threads.

The compiled form of a regular expression is not altered during matching, so the same compiled pattern can safely be used by several threads at once.

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(3) documentation. However, compiling a regular expression with one version of PCRE for use with a different version is not guaranteed to work and may cause crashes.

CHECKING BUILD-TIME OPTIONS

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(3) 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:

COMPILING A PATTERN

Either of the functions pcre_compile() or pcre_compile2() can be called to compile a pattern into an internal form. The only difference between the two interfaces is that pcre_compile2() has an additional argument, errorcodeptr, via which a numerical error code can be returned. To avoid too much repetition, we refer just to pcre_compile() below, but the information applies equally to pcre_compile2().

The pattern is a C string terminated by a binary zero, and is passed in the pattern argument. A pointer to a single block of memory that is obtained via pcre_malloc is returned. This contains the compiled code and related data. The pcre type is defined for the returned block; this is a typedef for a structure whose contents are not externally defined. It is up to the caller to free the memory (via pcre_free) when it is no longer required.

Although the compiled code of a PCRE regex is relocatable, that is, it does not depend on memory location, the complete pcre data block is not fully relocatable, because it may contain a copy of the tableptr argument, which is an address (see below).

The options argument contains various bit settings that affect the compilation. It should be zero if no options are required. The available options are described below. Some of them (in particular, those that are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see the detailed description in the pcrepattern(3) documentation). For those options that can be different in different parts of the pattern, the contents of the options argument specifies their settings at the start of compilation and execution. The PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and PCRE_NO_START_OPT options can be set at the time of matching as well as at compile time.

If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual error message. This is a static string that is part of the library. You must not try to free it. Normally, the offset from the start of the pattern to the byte that was being processed when the error was discovered is placed in the variable pointed to by erroffset, which must not be NULL (if it is, an immediate error is given). However, for an invalid UTF-8 string, the offset is that of the first byte of the failing character. Also, some errors are not detected until checks are carried out when the whole pattern has been scanned; in these cases the offset passed back is the length of the pattern.

Note that the offset is in bytes, not characters, even in UTF-8 mode. It may sometimes point into the middle of a UTF-8 character.

If pcre_compile2() is used instead of pcre_compile(), and the errorcodeptr argument is not NULL, a non-zero error code number is returned via this argument in the event of an error. This is in addition to the textual error message. Error codes and messages are listed below.

If the final argument, tableptr, is NULL, PCRE uses a default set of character tables that are built when PCRE is compiled, using the default C locale. Otherwise, tableptr must be an address that is the result of a call to pcre_maketables(). This value is stored with the compiled pattern, and used again by pcre_exec(), unless another table pointer is passed to it. For more discussion, see the section on locale support below.

This code fragment shows a typical straightforward call to pcre_compile():

 pcre *re;
 const char *error;
 int erroffset;
 re = pcre_compile(
   "^A.*Z",          /* the pattern */
   0,                /* default options */
   &error,           /* for error message */
   &erroffset,       /* for error offset */
   NULL);            /* use default character tables */

The following names for option bits are defined in the pcre.h header file:

Which characters are interpreted as newlines is controlled by the options passed to pcre_compile() or by a special sequence at the start of the pattern, as described in the section entitled "Newline conventions" in the pcrepattern documentation. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count.

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

(1) A lone closing square bracket in a pattern causes a compile-time error, because this is illegal in JavaScript (by default it is treated as a data character). Thus, the pattern AB]CD becomes illegal when this option is set.

(2) At run time, a back reference to an unset subpattern group matches an empty string (by default this causes the current matching alternative to fail). A pattern such as (\1)(a) succeeds when this option is set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility.

When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. If there are no newlines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect.

 PCRE_NEWLINE_CR
 PCRE_NEWLINE_LF
 PCRE_NEWLINE_CRLF
 PCRE_NEWLINE_ANYCRLF
 PCRE_NEWLINE_ANY

These options override the default newline definition that was chosen when PCRE was built. Setting the first or the second specifies that a newline is indicated by a single character (CR or LF, respectively). Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies that any of the three preceding sequences should be recognized. Setting PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be recognized. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). The last two are recognized only in UTF-8 mode.

The newline setting in the options word uses three bits that are treated as a number, giving eight possibilities. Currently only six are used (default plus the five values above). This means that if you set more than one newline option, the combination may or may not be sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and cause an error.

The only time that a line break in a pattern is specially recognized when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace characters, and so are ignored in this mode. Also, an unescaped # outside a character class indicates a comment that lasts until after the next line break sequence. In other circumstances, line break sequences in patterns are treated as literal data.

The newline option that is set at compile time becomes the default that is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

COMPILATION ERROR CODES

The following table lists the error codes than may be returned by pcre_compile2(), along with the error messages that may be returned by both compiling functions. As PCRE has developed, some error codes have fallen out of use. To avoid confusion, they have not been re-used.

  0  no error
  1  \ at end of pattern
  2  \c at end of pattern
  3  unrecognized character follows \
  4  numbers out of order in {} quantifier
  5  number too big in {} quantifier
  6  missing terminating ] for character class
  7  invalid escape sequence in character class
  8  range out of order in character class
  9  nothing to repeat
 10  [this code is not in use]
 11  internal error: unexpected repeat
 12  unrecognized character after (? or (?-
 13  POSIX named classes are supported only within a class
 14  missing )
 15  reference to non-existent subpattern
 16  erroffset passed as NULL
 17  unknown option bit(s) set
 18  missing ) after comment
 19  [this code is not in use]
 20  regular expression is too large
 21  failed to get memory
 22  unmatched parentheses
 23  internal error: code overflow
 24  unrecognized character after (?<
 25  lookbehind assertion is not fixed length
 26  malformed number or name after (?(
 27  conditional group contains more than two branches
 28  assertion expected after (?(
 29  (?R or (?[+-]digits must be followed by )
 30  unknown POSIX class name
 31  POSIX collating elements are not supported
 32  this version of PCRE is not compiled with PCRE_UTF8 support
 33  [this code is not in use]
 34  character value in \x{...} sequence is too large
 35  invalid condition (?(0)
 36  \C not allowed in lookbehind assertion
 37  PCRE does not support \L, \l, \N{name}, \U, or \u
 38  number after (?C is > 255
 39  closing ) for (?C expected
 40  recursive call could loop indefinitely
 41  unrecognized character after (?P
 42  syntax error in subpattern name (missing terminator)
 43  two named subpatterns have the same name
 44  invalid UTF-8 string
 45  support for \P, \p, and \X has not been compiled
 46  malformed \P or \p sequence
 47  unknown property name after \P or \p
 48  subpattern name is too long (maximum 32 characters)
 49  too many named subpatterns (maximum 10000)
 50  [this code is not in use]
 51  octal value is greater than \377 (not in UTF-8 mode)
 52  internal error: overran compiling workspace
 53  internal error: previously-checked referenced subpattern
       not found
 54  DEFINE group contains more than one branch
 55  repeating a DEFINE group is not allowed
 56  inconsistent NEWLINE options
 57  \g is not followed by a braced, angle-bracketed, or quoted
       name/number or by a plain number
 58  a numbered reference must not be zero
 59  an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
 60  (*VERB) not recognized
 61  number is too big
 62  subpattern name expected
 63  digit expected after (?+
 64  ] is an invalid data character in JavaScript compatibility mode
 65  different names for subpatterns of the same number are
       not allowed
 66  (*MARK) must have an argument
 67  this version of PCRE is not compiled with PCRE_UCP support
 68  \c must be followed by an ASCII character
 69  \k is not followed by a braced, angle-bracketed, or quoted name

The numbers 32 and 10000 in errors 48 and 49 are defaults; different values may be used if the limits were changed when PCRE was built.

STUDYING A PATTERN

If a compiled pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. The function pcre_study() takes a pointer to a compiled pattern as its first argument. If studying the pattern produces additional information that will help speed up matching, pcre_study() returns a pointer to a pcre_extra block, in which the study_data field points to the results of the study.

The returned value from pcre_study() can be passed directly to pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also contains other fields that can be set by the caller before the block is passed; these are described below in the section on matching a pattern.

If studying the pattern does not produce any useful information, pcre_study() returns NULL. In that circumstance, if the calling program wants to pass any of the other fields to pcre_exec() or pcre_dfa_exec(), it must set up its own pcre_extra block.

The second argument of pcre_study() contains option bits. 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 is set to point to a textual error message. This is a static string that is part of the library. You must not try to free it. You should test the error pointer for NULL after calling pcre_study(), to be sure that it has run successfully.

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 */

Studying a pattern does two things: first, a lower bound for the length of subject string that is needed to match the pattern is computed. This does not mean that there are any strings of that length that match, but it does guarantee that no shorter strings match. The value is used by pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to match strings that are shorter than the lower bound. You can find out the value in a calling program via the pcre_fullinfo() function.

Studying a pattern is also useful for non-anchored patterns that do not have a single fixed starting character. A bitmap of possible starting bytes is created. This speeds up finding a position in the subject at which to start matching.

The two optimizations just described can be disabled by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or pcre_dfa_exec(). You might want to do this if your pattern contains callouts or (*MARK), and you want to make use of these facilities in cases where matching fails. See the discussion of PCRE_NO_START_OPTIMIZE below.

LOCALE SUPPORT

PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables, indexed by character value. When running in UTF-8 mode, this applies only to characters with codes less than 128. By default, higher-valued codes never match escapes such as \w or \d, but they can be tested with \p if PCRE is built with Unicode character property support. Alternatively, the PCRE_UCP option can be set at compile time; this causes \w and friends to use Unicode property support instead of built-in tables. The use of locales with Unicode is discouraged. If you are handling characters with codes greater than 128, you should either use UTF-8 and Unicode, or use locales, but not try to mix the two.

PCRE contains an internal set of tables that are used when the final argument of pcre_compile() is NULL. These are sufficient for many applications. Normally, the internal tables recognize only ASCII characters. However, when PCRE is built, it is possible to cause the internal tables to be rebuilt in the default "C" locale of the local system, which may cause them to be different.

The internal tables can always be overridden by tables supplied by the application that calls PCRE. These may be created in a different locale from the default. As more and more applications change to using Unicode, the need for this locale support is expected to die away.

External tables are built by calling the pcre_maketables() function, which has no arguments, in the relevant locale. The result can then be passed to pcre_compile() or pcre_exec() as often as necessary. For example, to build and use tables that are appropriate for the French locale (where accented characters with values greater than 128 are treated as letters), the following code could be used:

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

The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name for the French locale is "french".

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

The pointer that is passed to pcre_compile() is saved with the compiled pattern, and the same tables are used via this pointer by pcre_study() and normally also by pcre_exec(). Thus, by default, for any single pattern, compilation, studying and matching all happen in the same locale, but different patterns can be compiled in different locales.

It is possible to pass a table pointer or NULL (indicating the use of the internal tables) to pcre_exec(). Although not intended for this purpose, this facility could be used to match a pattern in a different locale from the one in which it was compiled. Passing table pointers at run time is discussed below in the section on matching a pattern.

INFORMATION ABOUT A PATTERN

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;
 size_t 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:

If there is a fixed first byte, for example, from a pattern such as (cat|cow|coyote), its value is returned. 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.

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. Duplicate names may appear if (?| is used to create multiple groups with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern(3) page. Duplicate names for subpatterns with different numbers are permitted only if PCRE_DUPNAMES is set. In all cases of duplicate names, they appear in the table in the order in which they were found in the pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later subpatterns may have lower numbers.

As a simple example of the name/number table, consider the following pattern (assume PCRE_EXTENDED is set, so white space - including newlines - is ignored):

 (?<date> (?<year>(\d\d)?\d\d) -
 (?<month>\d\d) - (?<day>\d\d) )

There are four named subpatterns, so the table has four entries, and each entry in the table is eight bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes shown as ??:

 00 01 d  a  t  e  00 ??
 00 05 d  a  y  00 ?? ??
 00 04 m  o  n  t  h  00
 00 02 y  e  a  r  00 ??

When writing code to extract data from named subpatterns using the name-to-number map, remember that the length of the entries is likely to be different for each compiled pattern.

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().

OBSOLETE INFO FUNCTION

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).

REFERENCE COUNTS

The pcre_refcount() function is used to maintain a reference count in the data block that contains a compiled pattern. It is provided for the benefit of applications that operate in an object-oriented manner, where different parts of the application may be using the same compiled pattern, but you want to free the block when they are all done.

When a pattern is compiled, the reference count field is initialized to zero. It is changed only by calling this function, whose action is to add the adjust value (which may be positive or negative) to it. The yield of the function is the new value. However, the value of the count is constrained to lie between 0 and 65535, inclusive. If the new value is outside these limits, it is forced to the appropriate limit value.

Except when it is zero, the reference count is not correctly preserved if a pattern is compiled on one host and then transferred to a host whose byte-order is different. (This seems a highly unlikely scenario.)

MATCHING A PATTERN: THE TRADITIONAL FUNCTION

The function pcre_exec() is called to match a subject string against a compiled pattern, which is passed in the code argument. If the pattern was studied, the result of the study should be passed in the extra argument. This function is the main matching facility of the library, and it operates in a Perl-like manner. For specialist use there is also an alternative matching function, which is described below in the section about the pcre_dfa_exec() function.

In most applications, the pattern will have been compiled (and optionally studied) in the same process that calls pcre_exec(). However, it is possible to save compiled patterns and study data, and then use them later in different processes, possibly even on different hosts. For a discussion about this, see the pcreprecompile(3) documentation.

Here is an example of a simple call to pcre_exec():

 int rc;
 int ovector[30];
 rc = pcre_exec(
   re,             /* result of pcre_compile() */
   NULL,           /* we didn't study the pattern */
   "some string",  /* the subject string */
   11,             /* the length of the subject string */
   0,              /* start at offset 0 in the subject */
   0,              /* default options */
   ovector,        /* vector of integers for substring information */
   30);            /* number of elements (NOT size in bytes) */

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

 PCRE_EXTRA_STUDY_DATA
 PCRE_EXTRA_MATCH_LIMIT
 PCRE_EXTRA_MATCH_LIMIT_RECURSION
 PCRE_EXTRA_CALLOUT_DATA
 PCRE_EXTRA_TABLES
 PCRE_EXTRA_MARK

(*COMMIT)ABC

(*MARK:A)(X|Y)

 PCRE_PARTIAL_HARD
 PCRE_PARTIAL_SOFT

\Biss\B

How pcre_exec() returns captured substrings

In general, a pattern matches a certain portion of the subject, and in addition, further substrings from the subject may be picked out by parts of the pattern. Following the usage in Jeffrey Friedl's book, this is called "capturing" in what follows, and the phrase "capturing subpattern" is used for a fragment of a pattern that picks out a substring. PCRE supports several other kinds of parenthesized subpattern that do not cause substrings to be captured.

Captured substrings are returned to the caller via a vector of integers whose address is passed in ovector. The number of elements in the vector is passed in ovecsize, which must be a non-negative number.

	Note
	This argument is NOT the size of ovector in bytes.

The first two-thirds of the vector is used to pass back captured substrings, each substring using a pair of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching capturing subpatterns, and is not available for passing back information. The number passed in ovecsize should always be a multiple of three. If it is not, it is rounded down.

When a match is successful, information about captured substrings is returned in pairs of integers, starting at the beginning of ovector, and continuing up to two-thirds of its length at the most. The first element of each pair is set to the byte offset of the first character in a substring, and the second is set to the byte offset of the first character after the end of a substring.

	Note
	These values are always byte offsets, even in UTF-8 mode. They are not character counts.

The first pair of integers, ovector[0] and ovector[1], identify the portion of the subject string matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre_exec() is one more than the highest numbered pair that has been set. For example, if two substrings have been captured, the returned value is 3. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set.

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

If the vector is too small to hold all the captured substring offsets, it is used as far as possible (up to two-thirds of its length), and the function returns a value of zero. If 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.

The pcre_fullinfo() function can be used to find out how many capturing subpatterns there are in a compiled pattern. The smallest size for ovector that will allow for n captured substrings, in addition to the offsets of the substring matched by the whole pattern, is (n+1)*3.

It is possible for capturing subpattern number n+1 to match some part of the subject when subpattern n has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the return from the function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corresponding to unused subpatterns are set to -1.

Offset values that correspond to unused subpatterns at the end of the expression are also set to -1. For example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched. The return from the function is 2, because the highest used capturing subpattern number is 1, and the offsets for for the second and third capturing subpatterns (assuming the vector is large enough, of course) are set to -1.

	Note
	Elements of ovector that do not correspond to capturing parentheses in the pattern are never changed. That is, if a pattern contains n capturing parentheses, no more than ovector[0] to ovector[2n+1] are set by pcre_exec(). The other elements retain whatever values they previously had.

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

PCRE_ERROR_NOMATCH        (-1)

PCRE_ERROR_NULL           (-2)

PCRE_ERROR_BADOPTION      (-3)

PCRE_ERROR_BADMAGIC       (-4)

PCRE_ERROR_UNKNOWN_OPCODE (-5)

PCRE_ERROR_NOMEMORY       (-6)

PCRE_ERROR_NOSUBSTRING    (-7)

PCRE_ERROR_MATCHLIMIT     (-8)

PCRE_ERROR_CALLOUT        (-9)

PCRE_ERROR_BADUTF8        (-10)

PCRE_ERROR_BADUTF8_OFFSET (-11)

PCRE_ERROR_PARTIAL        (-12)

PCRE_ERROR_BADPARTIAL     (-13)

PCRE_ERROR_INTERNAL       (-14)

PCRE_ERROR_BADCOUNT       (-15)

PCRE_ERROR_RECURSIONLIMIT (-21)

PCRE_ERROR_BADNEWLINE     (-23)

PCRE_ERROR_BADOFFSET      (-24)

PCRE_ERROR_SHORTUTF8      (-25)

PCRE_ERROR_RECURSELOOP    (-26)

 PCRE_UTF8_ERR1
 PCRE_UTF8_ERR2
 PCRE_UTF8_ERR3
 PCRE_UTF8_ERR4
 PCRE_UTF8_ERR5

 PCRE_UTF8_ERR6
 PCRE_UTF8_ERR7
 PCRE_UTF8_ERR8
 PCRE_UTF8_ERR9
 PCRE_UTF8_ERR10

 PCRE_UTF8_ERR11
 PCRE_UTF8_ERR12

EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

Captured substrings can be accessed directly by using the offsets returned by pcre_exec() in ovector. For convenience, the functions pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() are provided for extracting captured substrings as new, separate, zero-terminated strings. These functions identify substrings by number. The next section describes functions for extracting named substrings.

A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result is not, of course, a C string. However, you can process such a string by referring to the length that is returned by pcre_copy_substring() and pcre_get_substring(). Unfortunately, the interface to pcre_get_substring_list() is not adequate for handling strings containing binary zeros, because the end of the final string is not independently indicated.

The first three arguments are the same for all three of these functions: subject is the subject string that has just been successfully matched, ovector is a pointer to the vector of integer offsets that was passed to pcre_exec(), and stringcount is the number of substrings that were captured by the match, including the substring that matched the entire regular expression. This is the value returned by pcre_exec() if it is greater than zero. If pcre_exec() returned zero, indicating that it ran out of space in ovector, the value passed as stringcount should be the number of elements in the vector divided by three.

The functions pcre_copy_substring() and pcre_get_substring() extract a single substring, whose number is given as stringnumber. A value of zero extracts the substring that matched the entire pattern, whereas higher values extract the captured substrings. For pcre_copy_substring(), the string is placed in buffer, whose length is given by buffersize, while for pcre_get_substring() a new block of memory is obtained via pcre_malloc, and its address is returned via stringptr. The yield of the function is the length of the string, not including the terminating zero, or one of these error codes:

PCRE_ERROR_NOMEMORY       (-6)

The buffer was too small for pcre_copy_substring(), or the attempt to get memory failed for pcre_get_substring().

PCRE_ERROR_NOSUBSTRING    (-7)

There is no substring whose number is stringnumber.

The pcre_get_substring_list() function extracts all available substrings and builds a list of pointers to them. All this is done in a single block of memory that is obtained via pcre_malloc. The address of the memory block is returned via listptr, which is also the start of the list of string pointers. The end of the list is marked by a NULL pointer. The yield of the function is zero if all went well, or the error code

PCRE_ERROR_NOMEMORY       (-6)

if the attempt to get the memory block failed.

When any of these functions encounter a substring that is unset, which can happen when capturing subpattern number n+1 matches some part of the subject, but subpattern n has not been used at all, they return an empty string. This can be distinguished from a genuine zero-length substring by inspecting the appropriate offset in ovector, which is negative for unset substrings.

The two convenience functions pcre_free_substring() and pcre_free_substring_list() can be used to free the memory returned by a previous call of pcre_get_substring() or pcre_get_substring_list(), respectively. They do nothing more than call the function pointed to by pcre_free, which of course could be called directly from a C program. However, PCRE is used in some situations where it is linked via a special interface to another programming language that cannot use pcre_free directly; it is for these cases that the functions are provided.

EXTRACTING CAPTURED SUBSTRINGS BY NAME

To extract a substring by name, you first have to find associated number. For example, for this pattern

(a+)b(?<xxx>\d+)...

the number of the subpattern called "xxx" is 2. If the name is known to be unique (PCRE_DUPNAMES was not set), you can find the number from the name by calling pcre_get_stringnumber(). The first argument is the compiled pattern, and the second is the name. The yield of the function is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no subpattern of that name.

Given the number, you can extract the substring directly, or use one of the functions described in the previous section. For convenience, there are also two functions that do the whole job.

Most of the arguments of pcre_copy_named_substring() and pcre_get_named_substring() are the same as those for the similarly named functions that extract by number. As these are described in the previous section, they are not re-described here. There are just two differences:

First, instead of a substring number, a substring name is given. Second, there is an extra argument, given at the start, which is a pointer to the compiled pattern. This is needed in order to gain access to the name-to-number translation table.

These functions call pcre_get_stringnumber(), and if it succeeds, they then call pcre_copy_substring() or pcre_get_substring(), as appropriate.

	Note
	If PCRE_DUPNAMES is set and there are duplicate names, the behaviour may not be what you want (see the next section).

Warning

If the pattern uses the (?| feature to set up multiple subpatterns with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern(3) page, you cannot use names to distinguish the different subpatterns, because names are not included in the compiled code. The matching process uses only numbers. For this reason, the use of different names for subpatterns of the same number causes an error at compile time.

DUPLICATE SUBPATTERN NAMES

When a pattern is compiled with the PCRE_DUPNAMES option, names for subpatterns are not required to be unique. (Duplicate names are always allowed for subpatterns with the same number, created by using the (?| feature. Indeed, if such subpatterns are named, they are required to use the same names.)

Normally, patterns with duplicate names are such that in any one match, only one of the named subpatterns participates. An example is shown in the pcrepattern(3) documentation.

When duplicates are present, pcre_copy_named_substring() and pcre_get_named_substring() return the first substring corresponding to the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is returned; no data is returned. The pcre_get_stringnumber() function returns one of the numbers that are associated with the name, but it is not defined which it is.

If you want to get full details of all captured substrings for a given name, you must use the pcre_get_stringtable_entries() function. The first argument is the compiled pattern, and the second is the name. The third and fourth are pointers to variables which are updated by the function. After it has run, they point to the first and last entries in the name-to-number table for the given name. The function itself returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if there are none. The format of the table is described above in the section entitled Information about a pattern above. Given all the relevant entries for the name, you can extract each of their numbers, and hence the captured data, if any.

FINDING ALL POSSIBLE MATCHES

The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match, starting at a given point in the subject. If you want to find all possible matches, or the longest possible match, consider using the alternative matching function (see below) instead. If you cannot use the alternative function, but still need to find all possible matches, you can kludge it up by making use of the callout facility, which is described in the pcrecallout(3) documentation.

What you have to do is to insert a callout right at the end of the pattern. When your callout function is called, extract and save the current matched substring. Then return 1, which forces pcre_exec() to backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.

MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

The function pcre_dfa_exec() is called to match a subject string against a compiled pattern, using a matching algorithm that scans the subject string just once, and does not backtrack. This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE patterns are not supported. Nevertheless, there are times when this kind of matching can be useful. For a discussion of the two matching algorithms, and a list of features that pcre_dfa_exec() does not support, see the pcrematching(3) documentation.

The arguments for the pcre_dfa_exec() function are the same as for pcre_exec(), plus two extras. The ovector argument is used in a different way, and this is described below. The other common arguments are used in the same way as for pcre_exec(), so their description is not repeated here.

The two additional arguments provide workspace for the function. The workspace vector should contain at least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More workspace will be needed for patterns and subjects where there are a lot of potential matches.

Here is an example of a simple call to pcre_dfa_exec():

 int rc;
 int ovector[10];
 int wspace[20];
 rc = pcre_dfa_exec(
   re,             /* result of pcre_compile() */
   NULL,           /* we didn't study the pattern */
   "some string",  /* the subject string */
   11,             /* the length of the subject string */
   0,              /* start at offset 0 in the subject */
   0,              /* default options */
   ovector,        /* vector of integers for substring information */
   10,             /* number of elements (NOT size in bytes) */
   wspace,         /* working space vector */
   20);            /* number of elements (NOT size in bytes) */

 PCRE_PARTIAL_HARD
 PCRE_PARTIAL_SOFT

<.*>

This is <something> <something else> <something further> no more

 <something>
 <something> <something else>
 <something> <something else> <something further>

PCRE_ERROR_DFA_UITEM      (-16)

PCRE_ERROR_DFA_UCOND      (-17)

PCRE_ERROR_DFA_UMLIMIT    (-18)

PCRE_ERROR_DFA_WSSIZE     (-19)

PCRE_ERROR_DFA_RECURSE    (-20)

SEE ALSO

pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.

REVISION

Last updated: 13 August 2011
Copyright (c) 1997-2011 University of Cambridge.