NAME

PCRE - Perl-compatible regular expressions

#include <pcre.h>

PCRE NATIVE API BASIC FUNCTIONS

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

pcre *pcre_compile2(const char *pattern, int options,
     int *errorcodeptr,
     const char **errptr, int *erroffset,
     const unsigned char *tableptr);

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

void pcre_free_study(pcre_extra *extra);

int pcre_exec(const pcre *code, const pcre_extra *extra,
     const char *subject, int length, int startoffset,
     int options, int *ovector, int ovecsize);

int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
     const char *subject, int length, int startoffset,
     int options, int *ovector, int ovecsize,
     int *workspace, int wscount);

PCRE NATIVE API STRING EXTRACTION FUNCTIONS

int pcre_copy_named_substring(const pcre *code,
     const char *subject, int *ovector,
     int stringcount, const char *stringname,
     char *buffer, int buffersize);

int pcre_copy_substring(const char *subject, int *ovector,
     int stringcount, int stringnumber, char *buffer,
     int buffersize);

int pcre_get_named_substring(const pcre *code,
     const char *subject, int *ovector,
     int stringcount, const char *stringname,
     const char **stringptr);

int pcre_get_stringnumber(const pcre *code,
     const char *name);

int pcre_get_stringtable_entries(const pcre *code,
     const char *name, char **first, char **last);

int pcre_get_substring(const char *subject, int *ovector,
     int stringcount, int stringnumber,
     const char **stringptr);

int pcre_get_substring_list(const char *subject,
     int *ovector, int stringcount, const char ***listptr);

void pcre_free_substring(const char *stringptr);

void pcre_free_substring_list(const char **stringptr);

PCRE NATIVE API AUXILIARY FUNCTIONS

int pcre_jit_exec(const pcre *code, const pcre_extra *extra,
     const char *subject, int length, int startoffset,
     int options, int *ovector, int ovecsize,
     pcre_jit_stack *jstack);

pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);

void pcre_jit_stack_free(pcre_jit_stack *stack);

void pcre_assign_jit_stack(pcre_extra *extra,
     pcre_jit_callback callback, void *data);

const unsigned char *pcre_maketables(void);

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

int pcre_refcount(pcre *code, int adjust);

int pcre_config(int what, void *where);

const char *pcre_version(void);

int pcre_pattern_to_host_byte_order(pcre *code,
     pcre_extra *extra, const unsigned char *tables);

PCRE NATIVE API INDIRECTED FUNCTIONS

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

void *(*pcre_stack_malloc)(size_t);

void (*pcre_stack_free)(void *);

int (*pcre_callout)(pcre_callout_block *);

int (*pcre_stack_guard)(void);

PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

As well as support for 8-bit character strings, PCRE also supports 16-bit strings (from release 8.30) and 32-bit strings (from release 8.32), by means of two additional libraries. They can be built as well as, or instead of, the 8-bit library. To avoid too much complication, this document describes the 8-bit versions of the functions, with only occasional references to the 16-bit and 32-bit libraries.

The 16-bit and 32-bit functions operate in the same way as their 8-bit counterparts; they just use different data types for their arguments and results, and their names start with pcre16_ or pcre32_ instead of pcre_. For every option that has UTF8 in its name (for example, PCRE_UTF8), there are corresponding 16-bit and 32-bit names with UTF8 replaced by UTF16 or UTF32, respectively. This facility is in fact just cosmetic; the 16-bit and 32-bit option names define the same bit values.

References to bytes and UTF-8 in this document should be read as references to 16-bit data units and UTF-16 when using the 16-bit library, or 32-bit data units and UTF-32 when using the 32-bit library, unless specified otherwise. More details of the specific differences for the 16-bit and 32-bit libraries are given in the pcre16 and pcre32 pages.

PCRE API OVERVIEW

PCRE has its own native API, which is described in this document. There are also some wrapper functions (for the 8-bit library only) that correspond to the POSIX regular expression API, but they do not give access to all the functionality. They are described in the pcreposix documentation. Both of these APIs define a set of C function calls. A C++ wrapper (again for the 8-bit library only) is also distributed with PCRE. It is documented in the pcrecpp page.

The native API C function prototypes are defined in the header file pcre.h, and on Unix-like systems the (8-bit) library itself is called libpcre. It can normally be accessed by adding -lpcre to the command for linking an application that uses PCRE. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release numbers for the library. Applications can use these to include support for different releases of PCRE.

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

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

Just-in-time compiler support is an optional feature of PCRE that can be built in appropriate hardware environments. It greatly speeds up the matching performance of many patterns. Simple programs can easily request that it be used if available, by setting an option that is ignored when it is not relevant. More complicated programs might need to make use of the functions pcre_jit_stack_alloc(), pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control the JIT code's memory usage.

From release 8.32 there is also a direct interface for JIT execution, which gives improved performance. The JIT-specific functions are discussed in the pcrejit documentation.

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

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

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

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

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

The function pcre_fullinfo() is used to find out information about a compiled pattern. The function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release.

The function pcre_refcount() maintains a reference count in a data block containing a compiled pattern. This is provided for the benefit of object-oriented applications.

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

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

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

The global variable pcre_stack_guard initially contains NULL. It can be set by the caller to a function that is called by PCRE whenever it starts to compile a parenthesized part of a pattern. When parentheses are nested, PCRE uses recursive function calls, which use up the system stack. This function is provided so that applications with restricted stacks can force a compilation error if the stack runs out. The function should return zero if all is well, or non-zero to force an error.

NEWLINES

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

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

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

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

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

MULTITHREADING

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

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

If the just-in-time optimization feature is being used, it needs separate memory stack areas for each thread. See the pcrejit documentation for more details.

SAVING PRECOMPILED PATTERNS FOR LATER USE

The compiled form of a regular expression can be saved and re-used at a later time, possibly by a different program, and even on a host other than the one on which it was compiled. Details are given in the pcreprecompile documentation, which includes a description of the pcre_pattern_to_host_byte_order() function. However, compiling a regular expression with one version of PCRE for use with a different version is not guaranteed to work and may cause crashes.

CHECKING BUILD-TIME OPTIONS

int pcre_config(int what, void *where);

The function pcre_config() makes it possible for a PCRE client to discover which optional features have been compiled into the PCRE library. The pcrebuild documentation has more details about these optional features.

The first argument for pcre_config() is an integer, specifying which information is required; the second argument is a pointer to a variable into which the information is placed. The returned value is zero on success, or the negative error code PCRE_ERROR_BADOPTION if the value in the first argument is not recognized. The following information is available:

PCRE_CONFIG_UTF8

The output is an integer that is set to one if UTF-8 support is available; otherwise it is set to zero. This value should normally be given to the 8-bit version of this function, pcre_config(). If it is given to the 16-bit or 32-bit version of this function, the result is PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UTF16

The output is an integer that is set to one if UTF-16 support is available; otherwise it is set to zero. This value should normally be given to the 16-bit version of this function, pcre16_config(). If it is given to the 8-bit or 32-bit version of this function, the result is PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UTF32

The output is an integer that is set to one if UTF-32 support is available; otherwise it is set to zero. This value should normally be given to the 32-bit version of this function, pcre32_config(). If it is given to the 8-bit or 16-bit version of this function, the result is PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UNICODE_PROPERTIES

The output is an integer that is set to one if support for Unicode character properties is available; otherwise it is set to zero.

PCRE_CONFIG_JIT

The output is an integer that is set to one if support for just-in-time compiling is available; otherwise it is set to zero.

PCRE_CONFIG_JITTARGET

The output is a pointer to a zero-terminated "const char *" string. If JIT support is available, the string contains the name of the architecture for which the JIT compiler is configured, for example "x86 32bit (little endian + unaligned)". If JIT support is not available, the result is NULL.

PCRE_CONFIG_NEWLINE

The output is an integer whose value specifies the default character sequence that is recognized as meaning "newline". The values that are supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC environments, CR, ANYCRLF, and ANY yield the same values. However, the value for LF is normally 21, though some EBCDIC environments use 37. The corresponding values for CRLF are 3349 and 3365. The default should normally correspond to the standard sequence for your operating system.

PCRE_CONFIG_BSR

The output is an integer whose value indicates what character sequences the \R escape sequence matches by default. A value of 0 means that \R matches any Unicode line ending sequence; a value of 1 means that \R matches only CR, LF, or CRLF. The default can be overridden when a pattern is compiled or matched.

PCRE_CONFIG_LINK_SIZE

The output is an integer that contains the number of bytes used for internal linkage in compiled regular expressions. For the 8-bit library, the value can be 2, 3, or 4. For the 16-bit library, the value is either 2 or 4 and is still a number of bytes. For the 32-bit library, the value is either 2 or 4 and is still a number of bytes. The default value of 2 is sufficient for all but the most massive patterns, since it allows the compiled pattern to be up to 64K in size. Larger values allow larger regular expressions to be compiled, at the expense of slower matching.

PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

The output is an integer that contains the threshold above which the POSIX interface uses malloc() for output vectors. Further details are given in the pcreposix documentation.

PCRE_CONFIG_PARENS_LIMIT

The output is a long integer that gives the maximum depth of nesting of parentheses (of any kind) in a pattern. This limit is imposed to cap the amount of system stack used when a pattern is compiled. It is specified when PCRE is built; the default is 250. This limit does not take into account the stack that may already be used by the calling application. For finer control over compilation stack usage, you can set a pointer to an external checking function in pcre_stack_guard.

PCRE_CONFIG_MATCH_LIMIT

The output is a long integer that gives the default limit for the number of internal matching function calls in a pcre_exec() execution. Further details are given with pcre_exec() below.

PCRE_CONFIG_MATCH_LIMIT_RECURSION

The output is a long integer that gives the default limit for the depth of recursion when calling the internal matching function in a pcre_exec() execution. Further details are given with pcre_exec() below.

PCRE_CONFIG_STACKRECURSE

The output is an integer that is set to one if internal recursion when running pcre_exec() is implemented by recursive function calls that use the stack to remember their state. This is the usual way that PCRE is compiled. The output is zero if PCRE was compiled to use blocks of data on the heap instead of recursive function calls. In this case, pcre_stack_malloc and pcre_stack_free are called to manage memory blocks on the heap, thus avoiding the use of the stack.

COMPILING A PATTERN

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

pcre *pcre_compile2(const char *pattern, int options,
     int *errorcodeptr,
     const char **errptr, int *erroffset,
     const unsigned char *tableptr);

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

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

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

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

If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual error message. This is a static string that is part of the library. You must not try to free it. Normally, the offset from the start of the pattern to the data unit 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 or UTF-16 string, the offset is that of the first data unit of the failing character.

Some errors are not detected until the whole pattern has been scanned; in these cases, the offset passed back is the length of the pattern. Note that the offset is in data units, not characters, even in a UTF mode. It may sometimes point into the middle of a UTF-8 or UTF-16 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() and pcre_dfa_exec() when the pattern is matched. For more discussion, see the section on locale support below.

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

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

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

PCRE_ANCHORED

If this bit is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.

PCRE_AUTO_CALLOUT

If this bit is set, pcre_compile() automatically inserts callout items, all with number 255, before each pattern item. For discussion of the callout facility, see the pcrecallout documentation.

PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE

These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. The default is specified when PCRE is built. It can be overridden from within the pattern, or by setting an option when a compiled pattern is matched.

PCRE_CASELESS

If this bit is set, letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support.

PCRE_DOLLAR_ENDONLY

If this bit is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. There is no equivalent to this option in Perl, and no way to set it within a pattern.

PCRE_DOTALL

If this bit is set, a dot metacharacter in the pattern matches a character of any value, including one that indicates a newline. However, it only ever matches one character, even if newlines are coded as CRLF. Without this option, a dot does not match when the current position is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A negative class such as [^a] always matches newline characters, independent of the setting of this option.

PCRE_DUPNAMES

If this bit is set, names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below; see also the pcrepattern documentation.

PCRE_EXTENDED

If this bit is set, most white space characters in the pattern are totally ignored except when escaped or inside a character class. However, white space is not allowed within sequences such as (?> that introduce various parenthesized subpatterns, nor within a numerical quantifier such as {1,3}. However, ignorable white space is permitted between an item and a following quantifier and between a quantifier and a following + that indicates possessiveness.

White space did not used to include the VT character (code 11), because Perl did not treat this character as white space. However, Perl changed at release 5.18, so PCRE followed at release 8.34, and VT is now treated as white space.

PCRE_EXTENDED also causes characters between an unescaped # outside a character class and the next newline, inclusive, to be ignored. PCRE_EXTENDED is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting.

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

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

PCRE_EXTRA

This option was invented in order to turn on additional functionality of PCRE that is incompatible with Perl, but it is currently of very little use. When set, any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. (Perl can, however, be persuaded to give an error for this, by running it with the -w option.) There are at present no other features controlled by this option. It can also be set by a (?X) option setting within a pattern.

PCRE_FIRSTLINE

If this option is set, an unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline.

PCRE_JAVASCRIPT_COMPAT

If this option is set, PCRE's behaviour is changed in some ways so that it is compatible with JavaScript rather than Perl. The changes are as follows:

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

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

(3) \U matches an upper case "U" character; by default \U causes a compile time error (Perl uses \U to upper case subsequent characters).

(4) \u matches a lower case "u" character unless it is followed by four hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, \u causes a compile time error (Perl uses it to upper case the following character).

(5) \x matches a lower case "x" character unless it is followed by two hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, as in Perl, a hexadecimal number is always expected after \x, but it may have zero, one, or two digits (so, for example, \xz matches a binary zero character followed by z).

PCRE_MULTILINE

By default, for the purposes of matching "start of line" and "end of line", 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, and the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (except when PCRE_DOLLAR_ENDONLY is set). Note, however, that unless PCRE_DOTALL is set, the "any character" metacharacter (.) does not match at a newline. This behaviour (for ^, $, and dot) is the same as Perl.

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

PCRE_NEVER_UTF

This option locks out interpretation of the pattern as UTF-8 (or UTF-16 or UTF-32 in the 16-bit and 32-bit libraries). In particular, it prevents the creator of the pattern from switching to UTF interpretation by starting the pattern with (*UTF). This may be useful in applications that process patterns from external sources. The combination of PCRE_UTF8 and PCRE_NEVER_UTF also causes an error.

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.

In an ASCII/Unicode environment, the Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). For the 8-bit library, the last two are recognized only in UTF-8 mode.

When PCRE is compiled to run in an EBCDIC (mainframe) environment, the code for CR is 0x0d, the same as ASCII. However, the character code for LF is normally 0x15, though in some EBCDIC environments 0x25 is used. Whichever of these is not LF is made to correspond to Unicode's NEL character. EBCDIC codes are all less than 256. For more details, see the pcrebuild documentation.

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

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

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

PCRE_NO_AUTO_CAPTURE

If this option is set, it disables the use of numbered capturing parentheses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl.

PCRE_NO_AUTO_POSSESS

If this option is set, it disables "auto-possessification". This is an optimization that, for example, turns a+b into a++b in order to avoid backtracks into a+ that can never be successful. However, if callouts are in use, auto-possessification means that some of them are never taken. You can set this option if you want the matching functions to do a full unoptimized search and run all the callouts, but it is mainly provided for testing purposes.

PCRE_NO_START_OPTIMIZE

This is an option that acts at matching time; that is, it is really an option for pcre_exec() or pcre_dfa_exec(). If it is set at compile time, it is remembered with the compiled pattern and assumed at matching time. This is necessary if you want to use JIT execution, because the JIT compiler needs to know whether or not this option is set. For details see the discussion of PCRE_NO_START_OPTIMIZE below.

PCRE_UCP

This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. By default, only ASCII characters are recognized, but if PCRE_UCP is set, Unicode properties are used instead to classify characters. More details are given in the section on generic character types in the pcrepattern page. If you set PCRE_UCP, matching one of the items it affects takes much longer. The option is available only if PCRE has been compiled with Unicode property support.

PCRE_UNGREEDY

This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern.

PCRE_UTF8

This option causes PCRE to regard both the pattern and the subject as strings of UTF-8 characters instead of single-byte strings. However, it is available only when PCRE is built to include UTF support. If not, the use of this option provokes an error. Details of how this option changes the behaviour of PCRE are given in the pcreunicode page.

PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is automatically checked. There is a discussion about the validity of UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is found, pcre_compile() returns an error. If you already know that your pattern is valid, and you want to skip this check for performance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of passing an invalid UTF-8 string as a pattern is undefined. It may cause your program to crash or loop. Note that this option can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity checking of subject strings only. If the same string is being matched many times, the option can be safely set for the second and subsequent matchings to improve performance.

COMPILATION ERROR CODES

The following table lists the error codes than may be returned by pcre_compile2(), along with the error messages that may be returned by both compiling functions. Note that error messages are always 8-bit ASCII strings, even in 16-bit or 32-bit mode. As PCRE has developed, some error codes have fallen out of use. To avoid confusion, they have not been re-used.

0 no error
1 \ at end of pattern
2 \c at end of pattern
3 unrecognized character follows \
4 numbers out of order in {} quantifier
5 number too big in {} quantifier
6 missing terminating ] for character class
7 invalid escape sequence in character class
8 range out of order in character class
9 nothing to repeat
10 [this code is not in use]
11 internal error: unexpected repeat
12 unrecognized character after (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NULL
17 unknown option bit(s) set
18 missing ) after comment
19 [this code is not in use]
20 regular expression is too large
21 failed to get memory
22 unmatched parentheses
23 internal error: code overflow
24 unrecognized character after (?<
25 lookbehind assertion is not fixed length
26 malformed number or name after (?(
27 conditional group contains more than two branches
28 assertion expected after (?(
29 (?R or (?[+-]digits must be followed by )
30 unknown POSIX class name
31 POSIX collating elements are not supported
32 this version of PCRE is compiled without UTF support
33 [this code is not in use]
34 character value in \x{} or \o{} is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N{name}, \U, or \u
38 number after (?C is > 255
39 closing ) for (?C expected
40 recursive call could loop indefinitely
41 unrecognized character after (?P
42 syntax error in subpattern name (missing terminator)
43 two named subpatterns have the same name
44 invalid UTF-8 string (specifically UTF-8)
45 support for \P, \p, and \X has not been compiled
46 malformed \P or \p sequence
47 unknown property name after \P or \p
48 subpattern name is too long (maximum 32 characters)
49 too many named subpatterns (maximum 10000)
50 [this code is not in use]
51 octal value is greater than \377 in 8-bit non-UTF-8 mode
52 internal error: overran compiling workspace
53 internal error: previously-checked referenced subpattern
not found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
60 (*VERB) not recognized or malformed
61 number is too big
62 subpattern name expected
63 digit expected after (?+
64 ] is an invalid data character in JavaScript compatibility mode
65 different names for subpatterns of the same number are
not allowed
66 (*MARK) must have an argument
67 this version of PCRE is not compiled with Unicode property
support
68 \c must be followed by an ASCII character
69 \k is not followed by a braced, angle-bracketed, or quoted name
70 internal error: unknown opcode in find_fixedlength()
71 \N is not supported in a class
72 too many forward references
73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
74 invalid UTF-16 string (specifically UTF-16)
75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
76 character value in \u.... sequence is too large
77 invalid UTF-32 string (specifically UTF-32)
78 setting UTF is disabled by the application
79 non-hex character in \x{} (closing brace missing?)
80 non-octal character in \o{} (closing brace missing?)
81 missing opening brace after \o
82 parentheses are too deeply nested
83 invalid range in character class
84 group name must start with a non-digit
85 parentheses are too deeply nested (stack check)

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

STUDYING A PATTERN

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

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

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

If studying the pattern does not produce any useful information, pcre_study() returns NULL by default. 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. However, if pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it returns a pcre_extra block even if studying did not find any additional information. It may still return NULL, however, if an error occurs in pcre_study().

The second argument of pcre_study() contains option bits. There are three further options in addition to PCRE_STUDY_EXTRA_NEEDED:

PCRE_STUDY_JIT_COMPILE
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

If any of these are set, and the just-in-time compiler is available, the pattern is further compiled into machine code that executes much faster than the pcre_exec() interpretive matching function. If the just-in-time compiler is not available, these options are ignored. All undefined bits in the options argument must be zero.

JIT compilation is a heavyweight optimization. It can take some time for patterns to be analyzed, and for one-off matches and simple patterns the benefit of faster execution might be offset by a much slower study time. Not all patterns can be optimized by the JIT compiler. For those that cannot be handled, matching automatically falls back to the pcre_exec() interpreter. For more details, see the pcrejit documentation.

The third argument for pcre_study() is a pointer for an error message. If studying succeeds (even if no data is returned), the variable it points to is set to NULL. Otherwise it is set to point to a textual error message. This is a static string that is part of the library. You must not try to free it. You should test the error pointer for NULL after calling pcre_study(), to be sure that it has run successfully.

When you are finished with a pattern, you can free the memory used for the study data by calling pcre_free_study(). This function was added to the API for release 8.20. For earlier versions, the memory could be freed with pcre_free(), just like the pattern itself. This will still work in cases where JIT optimization is not used, but it is advisable to change to the new function when convenient.

This is a typical way in which pcre_study() is used (except that in a real application there should be tests for errors):

int rc;
pcre *re;
pcre_extra *sd;
re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
sd = pcre_study(
re, /* result of pcre_compile() */
0, /* no options */
&error); /* set to NULL or points to a message */
rc = pcre_exec( /* see below for details of pcre_exec() options */
re, sd, "subject", 7, 0, 0, ovector, 30);
...
pcre_free_study(sd);
pcre_free(re);

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

Studying a pattern is also useful for non-anchored patterns that do not have a single fixed starting character. A bitmap of possible starting bytes is created. This speeds up finding a position in the subject at which to start matching. (In 16-bit mode, the bitmap is used for 16-bit values less than 256. In 32-bit mode, the bitmap is used for 32-bit values less than 256.)

These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), and the information is also used by the JIT compiler. The optimizations can be disabled by setting the PCRE_NO_START_OPTIMIZE option. 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.

PCRE_NO_START_OPTIMIZE can be specified at either compile time or execution time. However, if PCRE_NO_START_OPTIMIZE is passed to pcre_exec(), (that is, after any JIT compilation has happened) JIT execution is disabled. For JIT execution to work with PCRE_NO_START_OPTIMIZE, the option must be set at compile time.

There is a longer 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 code point. When running in UTF-8 mode, or in the 16- or 32-bit libraries, this applies only to characters with code points less than 256. By default, higher-valued code points never match escapes such as \w or \d. However, if PCRE is built with Unicode property support, all characters can be tested with \p and \P, or, alternatively, the PCRE_UCP option can be set when a pattern is compiled; this causes \w and friends to use Unicode property support instead of the built-in tables.

The use of locales with Unicode is discouraged. If you are handling characters with code points greater than 128, you should either use Unicode support, 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() 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 also by pcre_exec() and pcre_dfa_exec(). Thus, for any single pattern, compilation, studying and matching all happen in the same locale, but different patterns can be processed in different locales.

It is possible to pass a table pointer or NULL (indicating the use of the internal tables) to pcre_exec() or pcre_dfa_exec() (see the discussion below in the section on matching a pattern). This facility is provided for use with pre-compiled patterns that have been saved and reloaded. Character tables are not saved with patterns, so if a non-standard table was used at compile time, it must be provided again when the reloaded pattern is matched. Attempting to use this facility to match a pattern in a different locale from the one in which it was compiled is likely to lead to anomalous (usually incorrect) results.

INFORMATION ABOUT A PATTERN

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

The pcre_fullinfo() function returns information about a compiled pattern. It replaces the pcre_info() function, which was removed from the library at version 8.30, after more than 10 years of obsolescence.

The first argument for pcre_fullinfo() is a pointer to the compiled pattern. The second argument is the result of pcre_study(), or NULL if the pattern was not studied. The third argument specifies which piece of information is required, and the fourth argument is a pointer to a variable to receive the data. The yield of the function is zero for success, or one of the following negative numbers:

PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
endianness
PCRE_ERROR_BADOPTION the value of what was invalid
PCRE_ERROR_UNSET the requested field is not set

The "magic number" is placed at the start of each compiled pattern as an simple check against passing an arbitrary memory pointer. The endianness error can occur if a compiled pattern is saved and reloaded on a different host. Here is a typical call of pcre_fullinfo(), to obtain the length of the compiled pattern:

int rc;
size_t length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
sd, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length); /* where to put the data */

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

PCRE_INFO_BACKREFMAX

Return the number of the highest back reference in the pattern. The fourth argument should point to an int variable. Zero is returned if there are no back references.

PCRE_INFO_CAPTURECOUNT

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

PCRE_INFO_DEFAULT_TABLES

Return a pointer to the internal default character tables within PCRE. The fourth argument should point to an unsigned char * variable. This information call is provided for internal use by the pcre_study() function. External callers can cause PCRE to use its internal tables by passing a NULL table pointer.

PCRE_INFO_FIRSTBYTE (deprecated)

Return information about the first data unit of any matched string, for a non-anchored pattern. The name of this option refers to the 8-bit library, where data units are bytes. The fourth argument should point to an int variable. Negative values are used for special cases. However, this means that when the 32-bit library is in non-UTF-32 mode, the full 32-bit range of characters cannot be returned. For this reason, this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS and PCRE_INFO_FIRSTCHARACTER instead.

If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), its value is returned. In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library the value can be up to 0x10ffff.

If there is no fixed first value, and if either

(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern would be anchored),

-1 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise -2 is returned. For anchored patterns, -2 is returned.

PCRE_INFO_FIRSTCHARACTER

Return the value of the first data unit (non-UTF character) of any matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS returns 1; otherwise return 0. The fourth argument should point to an uint_t variable.

In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library in UTF-32 mode the value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32 mode.

PCRE_INFO_FIRSTCHARACTERFLAGS

Return information about the first data unit of any matched string, for a non-anchored pattern. The fourth argument should point to an int variable.

If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), 1 is returned, and the character value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no fixed first value, and if either

(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern would be anchored),

2 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise 0 is returned. For anchored patterns, 0 is returned.

PCRE_INFO_FIRSTTABLE

If the pattern was studied, and this resulted in the construction of a 256-bit table indicating a fixed set of values for the first data unit in any matching string, a pointer to the table is returned. Otherwise NULL is returned. The fourth argument should point to an unsigned char * variable.

PCRE_INFO_HASCRORLF

Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The fourth argument should point to an int variable. An explicit match is either a literal CR or LF character, or \r or \n.

PCRE_INFO_JCHANGED

Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The fourth argument should point to an int variable. (?J) and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

PCRE_INFO_JIT

Return 1 if the pattern was studied with one of the JIT options, and just-in-time compiling was successful. The fourth argument should point to an int variable. A return value of 0 means that JIT support is not available in this version of PCRE, or that the pattern was not studied with a JIT option, or that the JIT compiler could not handle this particular pattern. See the pcrejit documentation for details of what can and cannot be handled.

PCRE_INFO_JITSIZE

If the pattern was successfully studied with a JIT option, return the size of the JIT compiled code, otherwise return zero. The fourth argument should point to a size_t variable.

PCRE_INFO_LASTLITERAL

Return the value of the rightmost literal data unit that must exist in any matched string, other than at its start, if such a value has been recorded. The fourth argument should point to an int variable. If there is no such value, -1 is returned. For anchored patterns, a last literal value is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is "z", but for /^a\dz\d/ the returned value is -1.

Since for the 32-bit library using the non-UTF-32 mode, this function is unable to return the full 32-bit range of characters, this value is deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and PCRE_INFO_REQUIREDCHAR values should be used.

PCRE_INFO_MATCH_EMPTY

Return 1 if the pattern can match an empty string, otherwise 0. The fourth argument should point to an int variable.

PCRE_INFO_MATCHLIMIT

If the pattern set a match limit by including an item of the form (*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre_fullinfo() returns the error PCRE_ERROR_UNSET.

PCRE_INFO_MAXLOOKBEHIND

Return the number of characters (NB not data units) in the longest lookbehind assertion in the pattern. This information is useful when doing multi-segment matching using the partial matching facilities. Note that the simple assertions \b and \B require a one-character lookbehind. \A also registers a one-character lookbehind, though it does not actually inspect the previous character. This is to ensure that at least one character from the old segment is retained when a new segment is processed. Otherwise, if there are no lookbehinds in the pattern, \A might match incorrectly at the start of a new segment.

PCRE_INFO_MINLENGTH

If the pattern was studied and a minimum length for matching subject strings was computed, its value is returned. Otherwise the returned value is -1. The value is a number of characters, which in UTF mode may be different from the number of data units. The fourth argument should point to an int variable. A non-negative value is a lower bound to the length of any matching string. There may not be any strings of that length that do actually match, but every string that does match is at least that long.

PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE

PCRE supports the use of named as well as numbered capturing parentheses. The names are just an additional way of identifying the parentheses, which still acquire numbers. Several convenience functions such as pcre_get_named_substring() are provided for extracting captured substrings by name. It is also possible to extract the data directly, by first converting the name to a number in order to access the correct pointers in the output vector (described with pcre_exec() below). To do the conversion, you need to use the name-to-number map, which is described by these three values.

The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both of these return an int value. The entry size depends on the length of the longest name. PCRE_INFO_NAMETABLE returns a pointer to the first entry of the table. This is a pointer to char in the 8-bit library, where the first two bytes of each entry are the number of the capturing parenthesis, most significant byte first. In the 16-bit library, the pointer points to 16-bit data units, the first of which contains the parenthesis number. In the 32-bit library, the pointer points to 32-bit data units, the first of which contains the parenthesis number. The rest of the entry is the corresponding name, zero terminated.

The names are in alphabetical order. If (?| is used to create multiple groups with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern page, the groups may be given the same name, but there is only one entry in the table. Different names for groups of the same number are not permitted. Duplicate names for subpatterns with different numbers are permitted, but only if PCRE_DUPNAMES is set. They appear in the table in the order in which they were found in the pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later subpatterns may have lower numbers.

As a simple example of the name/number table, consider the following pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is set, so white space - including newlines - is ignored):

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

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

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

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

PCRE_INFO_OKPARTIAL

Return 1 if the pattern can be used for partial matching with pcre_exec(), otherwise 0. The fourth argument should point to an int variable. From release 8.00, this always returns 1, because the restrictions that previously applied to partial matching have been lifted. The pcrepartial documentation gives details of partial matching.

PCRE_INFO_OPTIONS

Return a copy of the options with which the pattern was compiled. The fourth argument should point to an unsigned long int variable. These option bits are those specified in the call to pcre_compile(), modified by any top-level option settings at the start of the pattern itself. In other words, they are the options that will be in force when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED.

A pattern is automatically anchored by PCRE if all of its top-level alternatives begin with one of the following:

^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears

For such patterns, the PCRE_ANCHORED bit is set in the options returned by pcre_fullinfo().

PCRE_INFO_RECURSIONLIMIT

If the pattern set a recursion limit by including an item of the form (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The fourth argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre_fullinfo() returns the error PCRE_ERROR_UNSET.

PCRE_INFO_SIZE

Return the size of the compiled pattern in bytes (for all three libraries). The fourth argument should point to a size_t variable. This value does not include the size of the pcre structure that is returned by pcre_compile(). The value that is passed as the argument to pcre_malloc() when pcre_compile() is getting memory in which to place the compiled data is the value returned by this option plus the size of the pcre structure. Studying a compiled pattern, with or without JIT, does not alter the value returned by this option.

PCRE_INFO_STUDYSIZE

Return the size in bytes (for all three libraries) of the data block pointed to by the study_data field in a pcre_extra block. If pcre_extra is NULL, or there is no study data, zero is returned. The fourth argument should point to a size_t variable. The study_data field is set by pcre_study() to record information that will speed up matching (see the section entitled "Studying a pattern" above). The format of the study_data block is private, but its length is made available via this option so that it can be saved and restored (see the pcreprecompile documentation for details).

PCRE_INFO_REQUIREDCHARFLAGS

Returns 1 if there is a rightmost literal data unit that must exist in any matched string, other than at its start. The fourth argument should point to an int variable. If there is no such value, 0 is returned. If returning 1, the character value itself can be retrieved using PCRE_INFO_REQUIREDCHAR.

For anchored patterns, a last literal value is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value 1 (with "z" returned from PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0.

PCRE_INFO_REQUIREDCHAR

Return the value of the rightmost literal data unit that must exist in any matched string, other than at its start, if such a value has been recorded. The fourth argument should point to an uint32_t variable. If there is no such value, 0 is returned.

REFERENCE COUNTS

int pcre_refcount(pcre *code, int adjust);

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

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

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

MATCHING A PATTERN: THE TRADITIONAL FUNCTION

int pcre_exec(const pcre *code, const pcre_extra *extra,
     const char *subject, int length, int startoffset,
     int options, int *ovector, int ovecsize);

The function pcre_exec() is called to match a subject string against a compiled pattern, which is passed in the code argument. If the pattern was studied, the result of the study should be passed in the extra argument. You can call pcre_exec() with the same code and extra arguments as many times as you like, in order to match different subject strings with the same pattern.

This function is the main matching facility of the library, and it operates in a Perl-like manner. For specialist use there is also an alternative matching function, which is described below in the section about the pcre_dfa_exec() function.

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

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

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

EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

int pcre_copy_substring(const char *subject, int *ovector,
     int stringcount, int stringnumber, char *buffer,
     int buffersize);

int pcre_get_substring(const char *subject, int *ovector,
     int stringcount, int stringnumber,
     const char **stringptr);

int pcre_get_substring_list(const char *subject,
     int *ovector, int stringcount, const char ***listptr);

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

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

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

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

PCRE_ERROR_NOMEMORY (-6)

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

PCRE_ERROR_NOSUBSTRING (-7)

There is no substring whose number is stringnumber.

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

PCRE_ERROR_NOMEMORY (-6)

if the attempt to get the memory block failed.

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

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

EXTRACTING CAPTURED SUBSTRINGS BY NAME

int pcre_get_stringnumber(const pcre *code,
     const char *name);

int pcre_copy_named_substring(const pcre *code,
     const char *subject, int *ovector,
     int stringcount, const char *stringname,
     char *buffer, int buffersize);

int pcre_get_named_substring(const pcre *code,
     const char *subject, int *ovector,
     int stringcount, const char *stringname,
     const char **stringptr);

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

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

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

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

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

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

These functions call pcre_get_stringnumber(), and if it succeeds, they then call pcre_copy_substring() or pcre_get_substring(), as appropriate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the behaviour may not be what you want (see the next section).

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

DUPLICATE SUBPATTERN NAMES

int pcre_get_stringtable_entries(const pcre *code,
     const char *name, char **first, char **last);

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

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

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

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

FINDING ALL POSSIBLE MATCHES

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

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

OBTAINING AN ESTIMATE OF STACK USAGE

Matching certain patterns using pcre_exec() can use a lot of process stack, which in certain environments can be rather limited in size. Some users find it helpful to have an estimate of the amount of stack that is used by pcre_exec(), to help them set recursion limits, as described in the pcrestack documentation. The estimate that is output by pcretest when called with the -m and -C options is obtained by calling pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its first five arguments.

Normally, if its first argument is NULL, pcre_exec() immediately returns the negative error code PCRE_ERROR_NULL, but with this special combination of arguments, it returns instead a negative number whose absolute value is the approximate stack frame size in bytes. (A negative number is used so that it is clear that no match has happened.) The value is approximate because in some cases, recursive calls to pcre_exec() occur when there are one or two additional variables on the stack.

If PCRE has been compiled to use the heap instead of the stack for recursion, the value returned is the size of each block that is obtained from the heap.

MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
     const char *subject, int length, int startoffset,
     int options, int *ovector, int ovecsize,
     int *workspace, int wscount);

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

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

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

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

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

SEE ALSO

pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).

AUTHOR

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

REVISION

Last updated: 18 December 2015
Copyright (c) 1997-2015 University of Cambridge.