# pcre specification

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- NAME
- SYNOPSIS
- DESCRIPTION
- MULTI-THREADING
- COMPILING A PATTERN
- STUDYING A PATTERN
- LOCALE SUPPORT
- INFORMATION ABOUT A PATTERN
- MATCHING A PATTERN
- EXTRACTING CAPTURED SUBSTRINGS
- LIMITATIONS
- DIFFERENCES FROM PERL
- REGULAR EXPRESSION DETAILS
- BACKSLASH
- CIRCUMFLEX AND DOLLAR
- FULL STOP (PERIOD, DOT)
- SQUARE BRACKETS
- POSIX CHARACTER CLASSES
- VERTICAL BAR
- INTERNAL OPTION SETTING
- SUBPATTERNS
- REPETITION
- BACK REFERENCES
- ASSERTIONS
- ONCE-ONLY SUBPATTERNS
- CONDITIONAL SUBPATTERNS
- COMMENTS
- RECURSIVE PATTERNS
- PERFORMANCE
- AUTHOR

- NAME

  pcre - Perl-compatible regular expressions.

- SYNOPSIS

  #include <pcre.h>

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

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

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

  int pcre_copy_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);

  const unsigned char *pcre_maketables(void);

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

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

  char *pcre_version(void);

  void *(*pcre_malloc)(size_t);

  void (*pcre_free)(void *);

- DESCRIPTION

  The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5, with just a few differences (see below). The current implementation corresponds to Perl 5.005, with some additional features from the Perl development release.

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

  The native API function prototypes are defined in the header file pcre.h, and on Unix systems the library itself is called libpcre.a, so can be accessed by adding -lpcre to the command for linking an application which calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release numbers for the library. Applications can use these to include support for different releases.

  The functions pcre_compile(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions, while pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() are convenience functions for extracting captured substrings from a matched subject string. The function pcre_maketables() is used (optionally) to build a set of character tables in the current locale for passing to pcre_compile().

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

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

- MULTI-THREADING

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

- COMPILING A PATTERN

  The function pcre_compile() is called to compile a pattern into an internal form. The pattern is a C string terminated by a binary zero, and is passed in the argument pattern. A pointer to a single block of memory that is obtained via pcre_malloc is returned. This contains the compiled code and related data. The pcre type is defined for this for convenience, but in fact pcre is just a typedef for void, since the contents of the block are not externally defined. It is up to the caller to free the memory when it is no longer required.

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

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

  If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual error message. The offset from the start of the pattern to the character where the error was discovered is placed in the variable pointed to by erroffset, which must not be NULL. If it is, an immediate error is given.

  If the final argument, tableptr, is NULL, PCRE uses a default set of character tables which are built when it is compiled, using the default C locale. Otherwise, tableptr must be the result of a call to pcre_maketables(). See the section on locale support below.

  The following option bits are defined in the header file:

        PCRE_ANCHORED

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

        PCRE_CASELESS

  If this bit is set, letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option.

        PCRE_DOLLAR_ENDONLY

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

        PCRE_DOTALL

  If this bit is set, a dot metacharater in the pattern matches all characters, including newlines. Without it, newlines are excluded. This option is equivalent to Perl's /s option. A negative class such as [^a] always matches a newline character, independent of the setting of this option.

        PCRE_EXTENDED

  If this bit is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x option, and makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.

        PCRE_EXTRA

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

        PCRE_MULTILINE

  By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.

  When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before any newline in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option. If there are no "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect.

        PCRE_UNGREEDY

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

- STUDYING A PATTERN

  When a pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. The function pcre_study() takes a pointer to a compiled pattern as its first argument, and returns a pointer to a pcre_extra block (another void typedef) containing additional information about the pattern; this can be passed to pcre_exec(). If no additional information is available, NULL is returned.

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

  The third argument for pcre_study() is a pointer to an error message. If studying succeeds (even if no data is returned), the variable it points to is set to NULL. Otherwise it points to a textual error message.

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

- LOCALE SUPPORT

  PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables. The library contains a default set of tables which is created in the default C locale when PCRE is compiled. This is used when the final argument of pcre_compile() is NULL, and is sufficient for many applications.

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

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

  The tables are built in memory that is obtained via pcre_malloc. The pointer that is passed to pcre_compile is saved with the compiled pattern, and the same tables are used via this pointer by pcre_study() and pcre_exec(). Thus for any single pattern, compilation, studying and matching all happen in the same locale, but different patterns can be compiled in different locales. It is the caller's responsibility to ensure that the memory containing the tables remains available for as long as it is needed.

- INFORMATION ABOUT A PATTERN

  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, while the fourth argument is a pointer to a variable to receive the data. The yield of the function is zero for success, or one of the following negative numbers:

        PCRE_ERROR_NULL       the argument code was NULL
                              the argument where was NULL
        PCRE_ERROR_BADMAGIC   the "magic number" was not found
        PCRE_ERROR_BADOPTION  the value of what was invalid

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

        PCRE_INFO_OPTIONS

  Return a copy of the options with which the pattern was compiled. The fourth argument should point to au unsigned long int variable. These option bits are those specified in the call to pcre_compile(), modified by any top-level option settings within the pattern itself, and with the PCRE_ANCHORED bit forcibly set if the form of the pattern implies that it can match only at the start of a subject string.

        PCRE_INFO_SIZE

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

        PCRE_INFO_CAPTURECOUNT

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

        PCRE_INFO_BACKREFMAX

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

        PCRE_INFO_FIRSTCHAR

  Return information about the first character of any matched string, for a non-anchored pattern. If there is a fixed first character, e.g. from a pattern such as (cat|cow|coyote), it is returned in the integer pointed to by where. Otherwise, if either

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

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

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

        PCRE_INFO_FIRSTTABLE

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

        PCRE_INFO_LASTLITERAL

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

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

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

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

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

- MATCHING A PATTERN

  The function pcre_exec() is called to match a subject string against a pre-compiled pattern, which is passed in the code argument. If the pattern has been studied, the result of the study should be passed in the extra argument. Otherwise this must be NULL.

  The PCRE_ANCHORED option can be passed in the options argument, whose unused bits must be zero. However, if a pattern was compiled with PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time.

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

        PCRE_NOTBOL

  The first character of the string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without PCRE_MULTILINE (at compile time) causes circumflex never to match.

        PCRE_NOTEOL

  The end of the string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never to match.

        PCRE_NOTEMPTY

  An empty string is not considered to be a valid match if this option is set. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern

        a?b?

  is applied to a string not beginning with "a" or "b", it matches the empty string at the start of the subject. With PCRE_NOTEMPTY set, this match is not valid, so PCRE searches further into the string for occurrences of "a" or "b".

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

  The subject string is passed as a pointer in subject, a length in length, and a starting offset in startoffset. Unlike the pattern string, it may contain binary zero characters. When the starting offset is zero, the search for a match starts at the beginning of the subject, and this is by far the most common case.

  A non-zero starting offset is useful when searching for another match in the same subject by calling pcre_exec() again after a previous success. Setting startoffset differs from just passing over a shortened string and setting PCRE_NOTBOL in the case of a pattern that begins with any kind of lookbehind. For example, consider the pattern

        \Biss\B

  which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the subject is not a word boundary.) When applied to the string "Mississipi" the first call to pcre_exec() finds the first occurrence. If pcre_exec() is called again with just the remainder of the subject, namely "issipi", it does not match, because \B is always false at the start of the subject, which is deemed to be a word boundary. However, if pcre_exec() is passed the entire string again, but with startoffset set to 4, it finds the second occurrence of "iss" because it is able to look behind the starting point to discover that it is preceded by a letter.

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

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

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

  When a match has been successful, information about captured substrings is returned in pairs of integers, starting at the beginning of ovector, and continuing up to two-thirds of its length at the most. The first element of a pair is set to the offset of the first character in a substring, and the second is set to the offset of the first character after the end of a substring. The first pair, ovector[0] and ovector[1], identify the portion of the subject string matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre_exec() is the number of pairs that have been set. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set.

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

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

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

  If the vector is too small to hold all the captured substrings, it is used as far as possible (up to two-thirds of its length), and the function returns a value of zero. In particular, if the substring offsets are not of interest, pcre_exec() may be called with ovector passed as NULL and ovecsize as zero. However, if the pattern contains back references and the ovector isn't big enough to remember the related substrings, PCRE has to get additional memory for use during matching. Thus it is usually advisable to supply an ovector.

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

  If pcre_exec() fails, it returns a negative number. The following are defined in the header file:

        PCRE_ERROR_NOMATCH        (-1)

  The subject string did not match the pattern.

        PCRE_ERROR_NULL           (-2)

  Either code or subject was passed as NULL, or ovector was NULL and ovecsize was not zero.

        PCRE_ERROR_BADOPTION      (-3)

  An unrecognized bit was set in the options argument.

        PCRE_ERROR_BADMAGIC       (-4)

  PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is passed a junk pointer. This is the error it gives when the magic number isn't present.

        PCRE_ERROR_UNKNOWN_NODE   (-5)

  While running the pattern match, an unknown item was encountered in the compiled pattern. This error could be caused by a bug in PCRE or by overwriting of the compiled pattern.

        PCRE_ERROR_NOMEMORY       (-6)

  If a pattern contains back references, but the ovector that is passed to pcre_exec() is not big enough to remember the referenced substrings, PCRE gets a block of memory at the start of matching to use for this purpose. If the call via pcre_malloc() fails, this error is given. The memory is freed at the end of matching.

- EXTRACTING CAPTURED SUBSTRINGS

  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. A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result does not, of course, function as a C string.

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

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

        PCRE_ERROR_NOMEMORY       (-6)

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

        PCRE_ERROR_NOSUBSTRING    (-7)

  There is no substring whose number is stringnumber.

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

        PCRE_ERROR_NOMEMORY       (-6)

  if the attempt to get the memory block failed.

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

- LIMITATIONS

  There are some size limitations in PCRE but it is hoped that they will never in practice be relevant. The maximum length of a compiled pattern is 65539 (sic) bytes. All values in repeating quantifiers must be less than 65536. The maximum number of capturing subpatterns is 99. The maximum number of all parenthesized subpatterns, including capturing subpatterns, assertions, and other types of subpattern, is 200.

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

- DIFFERENCES FROM PERL

  The differences described here are with respect to Perl 5.005.

  1. By default, a whitespace character is any character that the C library function isspace() recognizes, though it is possible to compile PCRE with alternative character type tables. Normally isspace() matches space, formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 no longer includes vertical tab in its set of whitespace characters. The \v escape that was in the Perl documentation for a long time was never in fact recognized. However, the character itself was treated as whitespace at least up to 5.002. In 5.004 and 5.005 it does not match \s.

  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times.

  3. Capturing subpatterns that occur inside negative lookahead assertions are counted, but their entries in the offsets vector are never set. Perl sets its numerical variables from any such patterns that are matched before the assertion fails to match something (thereby succeeding), but only if the negative lookahead assertion contains just one branch.

  4. Though binary zero characters are supported in the subject string, they are not allowed in a pattern string because it is passed as a normal C string, terminated by zero. The escape sequence "\0" can be used in the pattern to represent a binary zero.

  5. The following Perl escape sequences are not supported: \l, \u, \L, \U, \E, \Q. In fact these are implemented by Perl's general string-handling and are not part of its pattern matching engine.

  6. The Perl \G assertion is not supported as it is not relevant to single pattern matches.

  7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code}) constructions. However, there is some experimental support for recursive patterns using the non-Perl item (?R).

  8. There are at the time of writing some oddities in Perl 5.005_02 concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) are set.

  In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the future Perl changes to a consistent state that is different, PCRE may change to follow.

  9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.

  10. PCRE provides some extensions to the Perl regular expression facilities:

  (a) Although lookbehind assertions must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl 5.005 requires them all to have the same length.

  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- character matches only at the very end of the string.

  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted.

  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are.

  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start of the subject.

  (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for pcre_exec() have no Perl equivalents.

  (g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do this using the (?p{code}) construct, which PCRE cannot of course support.)

- REGULAR EXPRESSION DETAILS

  The syntax and semantics of the regular expressions supported by PCRE are described below. Regular expressions are also described in the Perl documentation and in a number of other books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly (ISBN 1-56592-257), covers them in great detail. The description here is intended as reference documentation.

  A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern

        The quick brown fox

  matches a portion of a subject string that is identical to itself. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of meta-characters, which do not stand for themselves but instead are interpreted in some special way.

  There are two different sets of meta-characters: those that are recognized anywhere in the pattern except within square brackets, and those that are recognized in square brackets. Outside square brackets, the meta-characters are as follows:

        \      general escape character with several uses
        ^      assert start of subject (or line, in multiline mode)
        $      assert end of subject (or line, in multiline mode)
        .      match any character except newline (by default)
        [      start character class definition
        |      start of alternative branch
        (      start subpattern
        )      end subpattern
        ?      extends the meaning of (
               also 0 or 1 quantifier
               also quantifier minimizer
        *      0 or more quantifier
        +      1 or more quantifier
        {      start min/max quantifier

  Part of a pattern that is in square brackets is called a "character class". In a character class the only meta-characters are:

        \      general escape character
        ^      negate the class, but only if the first character
        -      indicates character range
        ]      terminates the character class

  The following sections describe the use of each of the meta-characters.

- BACKSLASH

  The backslash character has several uses. Firstly, if it is followed by a non-alphameric character, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes.

  For example, if you want to match a "*" character, you write "\*" in the pattern. This applies whether or not the following character would otherwise be interpreted as a meta-character, so it is always safe to precede a non-alphameric with "\" to specify that it stands for itself. In particular, if you want to match a backslash, you write "\\".

  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the pattern (other than in a character class) and characters between a "#" outside a character class and the next newline character are ignored. An escaping backslash can be used to include a whitespace or "#" character as part of the pattern.

  A second use of backslash provides a way of encoding non-printing characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:

        \a     alarm, that is, the BEL character (hex 07)
        \cx    "control-x", where x is any character
        \e     escape (hex 1B)
        \f     formfeed (hex 0C)
        \n     newline (hex 0A)
        \r     carriage return (hex 0D)
        \t     tab (hex 09)
        \xhh   character with hex code hh
        \ddd   character with octal code ddd, or backreference

  The precise effect of "\cx" is as follows: if "x" is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.

  After "\x", up to two hexadecimal digits are read (letters can be in upper or lower case).

  After "\0" up to two further octal digits are read. In both cases, if there are fewer than two digits, just those that are present are used. Thus the sequence "\0\x\07" specifies two binary zeros followed by a BEL character. Make sure you supply two digits after the initial zero if the character that follows is itself an octal digit.

  The handling of a backslash followed by a digit other than 0 is complicated. Outside a character class, PCRE reads it and any following digits as a decimal number. If the number is less than 10, or if there have been at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference. A description of how this works is given later, following the discussion of parenthesized subpatterns.

  Inside a character class, or if the decimal number is greater than 9 and there have not been that many capturing subpatterns, PCRE re-reads up to three octal digits following the backslash, and generates a single byte from the least significant 8 bits of the value. Any subsequent digits stand for themselves. For example:

        \040   is another way of writing a space
        \40    is the same, provided there are fewer than 40
                  previous capturing subpatterns
        \7     is always a back reference
        \11    might be a back reference, or another way of
                  writing a tab
        \011   is always a tab
        \0113  is a tab followed by the character "3"
        \113   is the character with octal code 113 (since there
                  can be no more than 99 back references)
        \377   is a byte consisting entirely of 1 bits
        \81    is either a back reference, or a binary zero
                  followed by the two characters "8" and "1"

  Note that octal values of 100 or greater must not be introduced by a leading zero, because no more than three octal digits are ever read.

  All the sequences that define a single byte value can be used both inside and outside character classes. In addition, inside a character class, the sequence "\b" is interpreted as the backspace character (hex 08). Outside a character class it has a different meaning (see below).

  The third use of backslash is for specifying generic character types:

        \d     any decimal digit
        \D     any character that is not a decimal digit
        \s     any whitespace character
        \S     any character that is not a whitespace character
        \w     any "word" character
        \W     any "non-word" character

  Each pair of escape sequences partitions the complete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair.

  A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word". The definition of letters and digits is controlled by PCRE's character tables, and may vary if locale- specific matching is taking place (see "Locale support" above). For example, in the "fr" (French) locale, some character codes greater than 128 are used for accented letters, and these are matched by \w.

  These character type sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, since there is no character to match.

  The fourth use of backslash is for certain simple assertions. An assertion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are

        \b     word boundary
        \B     not a word boundary
        \A     start of subject (independent of multiline mode)
        \Z     end of subject or newline at end (independent of multiline mode)
        \z     end of subject (independent of multiline mode)

  These assertions may not appear in character classes (but note that "\b" has a different meaning, namely the backspace character, inside a character class).

  A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively.

  The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described below) in that they only ever match at the very start and end of the subject string, whatever options are set. They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argument of pcre_exec() is non-zero, \A can never match. The difference between \Z and \z is that \Z matches before a newline that is the last character of the string as well as at the end of the string, whereas \z matches only at the end.

- CIRCUMFLEX AND DOLLAR

  Outside a character class, in the default matching mode, the circumflex character is an assertion which is true only if the current matching point is at the start of the subject string. If the startoffset argument of pcre_exec() is non-zero, circumflex can never match. Inside a character class, circumflex has an entirely different meaning (see below).

  Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the subject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.)

  A dollar character is an assertion which is true only if the current matching point is at the end of the subject string, or immediately before a newline character that is the last character in the string (by default). Dollar need not be the last character of the pattern if a number of alternatives are involved, but it should be the last item in any branch in which it appears. Dollar has no special meaning in a character class.

  The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching time. This does not affect the \Z assertion.

  The meanings of the circumflex and dollar characters are changed if the PCRE_MULTILINE option is set. When this is the case, they match immediately after and immediately before an internal "\n" character, respectively, in addition to matching at the start and end of the subject string. For example, the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with "^" are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

  Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A is it always anchored, whether PCRE_MULTILINE is set or not.

- FULL STOP (PERIOD, DOT)

  Outside a character class, a dot in the pattern matches any one character in the subject, including a non-printing character, but not (by default) newline. If the PCRE_DOTALL option is set, dots match newlines as well. The handling of dot is entirely independent of the handling of circumflex and dollar, the only relationship being that they both involve newline characters. Dot has no special meaning in a character class.

- SQUARE BRACKETS

  An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not special. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash.

  A character class matches a single character in the subject; the character must be in the set of characters defined by the class, unless the first character in the class is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash.

  For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters which are in the class by enumerating those that are not. It is not an assertion: it still consumes a character from the subject string, and fails if the current pointer is at the end of the string.

  When caseless matching is set, any letters in a class represent both their upper case and lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would.

  The newline character is never treated in any special way in character classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class such as [^a] will always match a newline.

  The minus (hyphen) character can be used to specify a range of characters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class.

  It is not possible to have the literal character "]" as the end character of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is interpreted as a single class containing a range followed by two separate characters. The octal or hexadecimal representation of "]" can also be used to end a range.

  Ranges operate in ASCII collating sequence. They can also be used for characters specified numerically, for example [\000-\037]. If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\^_`wxyzabc], matched caselessly, and if character tables for the "fr" locale are in use, [\xc8-\xcb] matches accented E characters in both cases.

  The character types \d, \D, \s, \S, \w, and \W may also appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. A circumflex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore.

  All non-alphameric characters other than \, -, ^ (at the start) and the terminating ] are non-special in character classes, but it does no harm if they are escaped.

- POSIX CHARACTER CLASSES

  Perl 5.6 (not yet released at the time of writing) is going to support the POSIX notation for character classes, which uses names enclosed by [: and :] within the enclosing square brackets. PCRE supports this notation. For example,

        [01[:alpha:]%]

  matches "0", "1", any alphabetic character, or "%". The supported class names are

        alnum    letters and digits
        alpha    letters
        ascii    character codes 0 - 127
        cntrl    control characters
        digit    decimal digits (same as \d)
        graph    printing characters, excluding space
        lower    lower case letters
        print    printing characters, including space
        punct    printing characters, excluding letters and digits
        space    white space (same as \s)
        upper    upper case letters
        word     "word" characters (same as \w)
        xdigit   hexadecimal digits

  The names "ascii" and "word" are Perl extensions. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example,

        [12[:^digit:]]

  matches "1", "2", or any non-digit. PCRE (and Perl) also recogize the POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered.

- VERTICAL BAR

  Vertical bar characters are used to separate alternative patterns. For example, the pattern

        gilbert|sullivan

  matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern.

- INTERNAL OPTION SETTING

  The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are

        i  for PCRE_CASELESS
        m  for PCRE_MULTILINE
        s  for PCRE_DOTALL
        x  for PCRE_EXTENDED

  For example, (?im) sets caseless, multiline matching. It is also possible to unset these options by preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. If a letter appears both before and after the hyphen, the option is unset.

  The scope of these option changes depends on where in the pattern the setting occurs. For settings that are outside any subpattern (defined below), the effect is the same as if the options were set or unset at the start of matching. The following patterns all behave in exactly the same way:

        (?i)abc
        a(?i)bc
        ab(?i)c
        abc(?i)

  which in turn is the same as compiling the pattern abc with PCRE_CASELESS set. In other words, such "top level" settings apply to the whole pattern (unless there are other changes inside subpatterns). If there is more than one setting of the same option at top level, the rightmost setting is used.

  If an option change occurs inside a subpattern, the effect is different. This is a change of behaviour in Perl 5.005. An option change inside a subpattern affects only that part of the subpattern that follows it, so

        (a(?i)b)c

  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same subpattern. For example,

        (a(?i)b|c)

  matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings happen at compile time. There would be some very weird behaviour otherwise.

  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the same way as the Perl-compatible options by using the characters U and X respectively. The (?X) flag setting is special in that it must always occur earlier in the pattern than any of the additional features it turns on, even when it is at top level. It is best put at the start.

- SUBPATTERNS

  Subpatterns are delimited by parentheses (round brackets), which can be nested. Marking part of a pattern as a subpattern does two things:

  1. It localizes a set of alternatives. For example, the pattern

        cat(aract|erpillar|)

  matches one of the words "cat", "cataract", or "caterpillar". Without the parentheses, it would match "cataract", "erpillar" or the empty string.

  2. It sets up the subpattern as a capturing subpattern (as defined above). When the whole pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller via the ovector argument of pcre_exec(). Opening parentheses are counted from left to right (starting from 1) to obtain the numbers of the capturing subpatterns.

  For example, if the string "the red king" is matched against the pattern

        the ((red|white) (king|queen))

  the captured substrings are "red king", "red", and "king", and are numbered 1, 2, and 3.

  The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by "?:", the subpattern does not do any capturing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the pattern

        the ((?:red|white) (king|queen))

  the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of captured substrings is 99, and the maximum number of all subpatterns, both capturing and non-capturing, is 200.

  As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns

        (?i:saturday|sunday)
        (?:(?i)saturday|sunday)

  match exactly the same set of strings. Because alternative branches are tried from left to right, and options are not reset until the end of the subpattern is reached, an option setting in one branch does affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday".

- REPETITION

  Repetition is specified by quantifiers, which can follow any of the following items:

        a single character, possibly escaped
        the . metacharacter
        a character class
        a back reference (see next section)
        a parenthesized subpattern (unless it is an assertion - see below)

  The general repetition quantifier specifies a minimum and maximum number of permitted matches, by giving the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example:

        z{2,4}

  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit; if the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus

        [aeiou]{3,}

  matches at least 3 successive vowels, but may match many more, while

        \d{8}

  matches exactly 8 digits. An opening curly bracket that appears in a position where a quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For example, {,6} is not a quantifier, but a literal string of four characters.

  The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present.

  For convenience (and historical compatibility) the three most common quantifiers have single-character abbreviations:

        *    is equivalent to {0,}
        +    is equivalent to {1,}
        ?    is equivalent to {0,1}

  It is possible to construct infinite loops by following a subpattern that can match no characters with a quantifier that has no upper limit, for example:

        (a?)*

  Earlier versions of Perl and PCRE used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but if any repetition of the subpattern does in fact match no characters, the loop is forcibly broken.

  By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted times), without causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between the sequences /* and */ and within the sequence, individual * and / characters may appear. An attempt to match C comments by applying the pattern

        /\*.*\*/

  to the string

        /* first command */  not comment  /* second comment */

  fails, because it matches the entire string due to the greediness of the .* item.

  However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the pattern

        /\*.*?\*/

  does the right thing with the C comments. The meaning of the various quantifiers is not otherwise changed, just the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in

        \d??\d

  which matches one digit by preference, but can match two if that is the only way the rest of the pattern matches.

  If the PCRE_UNGREEDY option is set (an option which is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour.

  When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more store is required for the compiled pattern, in proportion to the size of the minimum or maximum.

  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent to Perl's /s) is set, thus allowing the . to match newlines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first. PCRE treats such a pattern as though it were preceded by \A. In cases where it is known that the subject string contains no newlines, it is worth setting PCRE_DOTALL when the pattern begins with .* in order to obtain this optimization, or alternatively using ^ to indicate anchoring explicitly.

  When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration. For example, after

        (tweedle[dume]{3}\s*)+

  has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured values may have been set in previous iterations. For example, after

        /(a|(b))+/

  matches "aba" the value of the second captured substring is "b".

- BACK REFERENCES

  Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing subpattern earlier (i.e. to its left) in the pattern, provided there have been that many previous capturing left parentheses.

  However, if the decimal number following the backslash is less than 10, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pattern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 10. See the section entitled "Backslash" above for further details of the handling of digits following a backslash.

  A back reference matches whatever actually matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself. So the pattern

        (sens|respons)e and \1ibility

  matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the back reference, the case of letters is relevant. For example,

        ((?i)rah)\s+\1

  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly.

  There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, any back references to it always fail. For example, the pattern

        (a|(bc))\2

  always fails if it starts to match "a" rather than "bc". Because there may be up to 99 back references, all digits following the backslash are taken as part of a potential back reference number. If the pattern continues with a digit character, some delimiter must be used to terminate the back reference. If the PCRE_EXTENDED option is set, this can be whitespace. Otherwise an empty comment can be used.

  A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated subpatterns. For example, the pattern

        (a|b\1)+

  matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of the subpattern, the back reference matches the character string corresponding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero.

- ASSERTIONS

  An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \Z, \z, ^ and $ are described above. More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it.

  An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed. Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example,

        \w+(?=;)

  matches a word followed by a semicolon, but does not include the semicolon in the match, and

        foo(?!bar)

  matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern

        (?!foo)bar

  does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve this effect.

  Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example,

        (?<!foo)bar

  does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are several alternatives, they do not all have to have the same fixed length. Thus

        (?<=bullock|donkey)

  is permitted, but

        (?<!dogs?|cats?)

  causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl 5.005, which requires all branches to match the same length of string. An assertion such as

        (?<=ab(c|de))

  is not permitted, because its single top-level branch can match two different lengths, but it is acceptable if rewritten to use two top-level branches:

        (?<=abc|abde)

  The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position back by the fixed width and then try to match. If there are insufficient characters before the current position, the match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns can be particularly useful for matching at the ends of strings; an example is given at the end of the section on once-only subpatterns.

  Several assertions (of any sort) may occur in succession. For example,

        (?<=\d{3})(?<!999)foo

  matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" preceded by six characters, the first of which are digits and the last three of which are not "999". For example, it doesn't match "123abcfoo". A pattern to do that is

        (?<=\d{3}...)(?<!999)foo

  This time the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks that the preceding three characters are not "999".

  Assertions can be nested in any combination. For example,

        (?<=(?<!foo)bar)baz

  matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while

        (?<=\d{3}(?!999)...)foo

  is another pattern which matches "foo" preceded by three digits and any three characters that are not "999".

  Assertion subpatterns are not capturing subpatterns, and may not be repeated, because it makes no sense to assert the same thing several times. If any kind of assertion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing subpatterns in the whole pattern. However, substring capturing is carried out only for positive assertions, because it does not make sense for negative assertions.

  Assertions count towards the maximum of 200 parenthesized subpatterns.

- ONCE-ONLY SUBPATTERNS

  With both maximizing and minimizing repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on.

  Consider, for example, the pattern \d+foo when applied to the subject line

        123456bar

  After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. Once-only subpatterns provide the means for specifying that once a portion of the pattern has matched, it is not to be re-evaluated in this way, so the matcher would give up immediately on failing to match "foo" the first time. The notation is another kind of special parenthesis, starting with (?> as in this example:

        (?>\d+)bar

  This kind of parenthesis "locks up" the part of the pattern it contains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal.

  An alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string.

  Once-only subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

  This construction can of course contain arbitrarily complicated subpatterns, and it can be nested.

  Once-only subpatterns can be used in conjunction with lookbehind assertions to specify efficient matching at the end of the subject string. Consider a simple pattern such as

        abcd$

  when applied to a long string which does not match. Because matching proceeds from left to right, PCRE will look for each "a" in the subject and then see if what follows matches the rest of the pattern. If the pattern is specified as

        ^.*abcd$

  the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as

        ^(?>.*)(?<=abcd)

  there can be no backtracking for the .* item; it can match only the entire string. The subsequent lookbehind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time.

  When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of a once-only subpattern is the only way to avoid some failing matches taking a very long time indeed. The pattern

        (\D+|<\d+>)*[!?]

  matches an unlimited number of substrings that either consist of non-digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to

        aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

  it takes a long time before reporting failure. This is because the string can be divided between the two repeats in a large number of ways, and all have to be tried. (The example used [!?] rather than a single character at the end, because both PCRE and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single character that is required for a match, and fail early if it is not present in the string.) If the pattern is changed to

        ((?>\D+)|<\d+>)*[!?]

  sequences of non-digits cannot be broken, and failure happens quickly.

- CONDITIONAL SUBPATTERNS

  It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a previous capturing subpattern matched or not. The two possible forms of conditional subpattern are

        (?(condition)yes-pattern)
        (?(condition)yes-pattern|no-pattern)

  If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If there are more than two alternatives in the subpattern, a compile-time error occurs.

  There are two kinds of condition. If the text between the parentheses consists of a sequence of digits, the condition is satisfied if the capturing subpattern of that number has previously matched. Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE_EXTENDED option) and to divide it into three parts for ease of discussion:

        ( \( )?    [^()]+    (?(1) \) )

  The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The second part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether the first set of parentheses matched or not. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a closing parenthesis is required. Otherwise, since no-pattern is not present, the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses.

  If the condition is not a sequence of digits, it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line:

        (?(?=[^a-z]*[a-z])
        \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

  The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

- COMMENTS

  The sequence (?# marks the start of a comment which continues up to the next closing parenthesis. Nested parentheses are not permitted. The characters that make up a comment play no part in the pattern matching at all.

  If the PCRE_EXTENDED option is set, an unescaped # character outside a character class introduces a comment that continues up to the next newline character in the pattern.

- RECURSIVE PATTERNS

  Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth. Perl 5.6 has provided an experimental facility that allows regular expressions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern to solve the parentheses problem can be created like this:

        $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

  The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. Obviously, PCRE cannot support the interpolation of Perl code. Instead, the special item (?R) is provided for the specific case of recursion. This PCRE pattern solves the parentheses problem (assume the PCRE_EXTENDED option is set so that white space is ignored):

        \( ( (?>[^()]+) | (?R) )* \)

  First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (i.e. a correctly parenthesized substring). Finally there is a closing parenthesis.

  This particular example pattern contains nested unlimited repeats, and so the use of a once-only subpattern for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when it is applied to

        (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

  it yields "no match" quickly. However, if a once-only subpattern is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported.

  The values set for any capturing subpatterns are those from the outermost level of the recursion at which the subpattern value is set. If the pattern above is matched against

        (ab(cd)ef)

  the value for the capturing parentheses is "ef", which is the last value taken on at the top level. If additional parentheses are added, giving

        \( ( ( (?>[^()]+) | (?R) )* ) \)
           ^                        ^
           ^                        ^

  the string they capture is "ab(cd)ef", the contents of the top level parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory to store data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free afterwards. If no memory can be obtained, it saves data for the first 15 capturing parentheses only, as there is no way to give an out-of-memory error from within a recursion.
- PERFORMANCE

  Certain items that may appear in patterns are more efficient than others. It is more efficient to use a character class like [aeiou] than a set of alternatives such as (a|e|i|o|u). In general, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of discussion about optimizing regular expressions for efficient performance.

  When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is implicitly anchored by PCRE, since it can match only at the start of a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization, because the . metacharacter does not then match a newline, and if the subject string contains newlines, the pattern may match from the character immediately following one of them instead of from the very start. For example, the pattern

        (.*) second

  matches the subject "first\nand second" (where \n stands for a newline character) with the first captured substring being "and". In order to do this, PCRE has to retry the match starting after every newline in the subject.

  If you are using such a pattern with subject strings that do not contain newlines, the best performance is obtained by setting PCRE_DOTALL, or starting the pattern with ^.* to indicate explicit anchoring. That saves PCRE from having to scan along the subject looking for a newline to restart at.

  Beware of patterns that contain nested indefinite repeats. These can take a long time to run when applied to a string that does not match. Consider the pattern fragment

        (a+)*

  This can match "aaaa" in 33 different ways, and this number increases very rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 times, and for each of those cases other than 0, the + repeats can match different numbers of times.) When the remainder of the pattern is such that the entire match is going to fail, PCRE has in principle to try every possible variation, and this can take an extremely long time.

  An optimization catches some of the more simple cases such as

        (a+)*b

  where a literal character follows. Before embarking on the standard matching procedure, PCRE checks that there is a "b" later in the subject string, and if there is not, it fails the match immediately. However, when there is no following literal this optimization cannot be used. You can see the difference by comparing the behaviour of

        (a+)*\d

  with the pattern above. The former gives a failure almost instantly when applied to a whole line of "a" characters, whereas the latter takes an appreciable time with strings longer than about 20 characters.

- AUTHOR

  Philip Hazel <ph10@cam.ac.uk>
  University Computing Service,
  New Museums Site,
  Cambridge CB2 3QG, England.
  Phone: +44 1223 334714

  Last updated: 27 January 2000
  Copyright (c) 1997-2000 University of Cambridge.