PCRE(3)

NAME
pcre - Perl-compatible regular expressions.

SYNOPSIS
#include <pcre.h>

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

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 options,
int *ovector, int ovecsize);

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

char *pcre_version(void);

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

unsigned char *pcre_cbits[128];

unsigned char *pcre_ctypes[256];

unsigned char *pcre_fcc[256];

unsigned char *pcre_lcc[256];

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 5.004. PCRE has its own native API, which is described in this man page. There is also a set of wrapper functions that correspond to the POSIX API. See pcreposix (3). The three functions pcre_compile(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions. The function pcre_info() is used to find out information about a compiled pattern, while the function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release. The global variables pcre_malloc and pcre_free initially contain the entry points of the standard malloc() and free() functions respectively. PCRE calls the memory management functions via these variables, so a calling program can replace them if it wishes to intercept the calls. This should be done before calling any PCRE functions. The other global variables are character tables. They are initialized when PCRE is compiled, from source that is generated by reference to the C character type functions, but which the maintainer of PCRE is free to modify. In principle they could also be modified at runtime. See PCRE's README file for more details.

MULTI-THREADING
The PCRE functions can be used in multi-threading applications, with the proviso that the character tables and the memory management functions pointed to by pcre_malloc and pcre_free will be 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 the compiled code block is returned. 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 defined.

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. Those options that are compabible with Perl can also be set at compile time from within the pattern (see the detailed description of regular expressions below) and all options except PCRE_EXTENDED, PCRE_EXTRA and PCRE_UNGREEDY can be set at the time of matching.

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.

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 in any subject string. 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. By default, it 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. By default, 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 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.
  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. 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_EXTRA This option turns on additional functionality of PCRE that is incompatible with Perl. 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 two extra features currently provided, and both are in some sense experimental additions that are useful for influencing the progress of a match.
  (1) The sequence \X inserts a Prolog-like "cut" into the expression.
  (2) Once a subpattern enclosed in (?>subpat) brackets has matched,
      backtracking never goes back into the pattern. See below for further details of both of these. PCRE_EXTRA can be set by a (?X) option setting within the pattern, but this must precede anything in the pattern which relies on its being set.
  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. The only one currently supported is PCRE_CASELESS. It forces the studying to be done in a caseless manner, even if the original pattern was compiled without PCRE_CASELESS. When the result of pcre_study() is passed to pcre_exec(), it is used only if its caseless state is the same as that of the matching process. A pattern that is compiled without PCRE_CASELESS can be studied with and without PCRE_CASELESS, and the appropriate data passed to pcre_exec() with and without the PCRE_CASELESS flag. 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.

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 subject string is passed as a pointer in subject and a length in length. Unlike the pattern string, it may contain binary zero characters. The options PCRE_ANCHORED, PCRE_CASELESS, PCRE_DOLLAR_ENDONLY, PCRE_DOTALL, and PCRE_MULTILINE can be passed in the options argument, whose unused bits must be zero. However, if a pattern is compiled with any of these options, they cannot be unset when it is obeyed. There are also two 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 either compile or match 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. Setting this without PCRE_MULTILINE (at either compile or match time) causes dollar never to match. 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. This should always be an even number, because the elements are used in pairs. If an odd number is passed, it is rounded down. 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. 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, 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. 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_BADREF (-2) There was a back-reference in the pattern to a capturing subpattern that had not previously been set.
  PCRE_ERROR_NULL (-3) Either code or subject was passed as NULL, or ovector was NULL and ovecsize was not zero.
  PCRE_ERROR_BADOPTION (-4) An unrecognized bit was set in the options argument.
  PCRE_ERROR_BADMAGIC (-5) 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 (-6) 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 (-7) 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.

INFORMATION ABOUT A PATTERN
The pcre_info() function returns information about a compiled pattern. Its yield 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. If the firstcharptr argument is not NULL, is is used to pass back information about the first character of any matched string. If there is a fixed first character, e.g. from a pattern such as (cat|cow|coyote), then it is returned in the integer pointed to by firstcharptr. Otherwise, if the pattern was compiled with the PCRE_MULTILINE option, and every branch started with "^", then -1 is returned, indicating that the pattern will match at the start of a subject string or after any "\n" within the string. Otherwise -2 is returned.

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 and assertions, 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 5.004. 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 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. If a backreference can never be matched, PCRE diagnoses an error. In a case like
  /(123)\2/ the error occurs at compile time. Perl gives no compile time error; version 5.004 either always fails to match, or gives a segmentation fault at runtime. In more complicated cases such as
  /(1)(2)(3)(4)(5)(6)(7)(8)(9)(10\10)/ PCRE returns PCRE_ERROR_BADREF at run time. Perl always fails to match. 8. PCRE provides some extensions to the Perl regular expression facilities: (a) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- character matches only at the very end of the string. (b) If PCRE_EXTRA is set, the \X assertion (a Prolog-like "cut") is recognized, and a backslash followed by a letter with no special meaning is faulted. There is also a new kind of parenthesized subpattern starting with (?> which has a block on backtracking into it once it has matched. (c) 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.

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 1-56592-257-3), 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 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 if the character that follows could otherwise be taken as another 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". 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 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 backslashed assertions are
  \b word boundary
  \B not a word boundary
  \A start of subject (independent of multiline mode)
  \Z end of subject (independent of multiline mode) 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. More complicated assertions are also supported (see below). The "\A" and "\Z" assertions differ from the traditional "^" and "$" (described below) in that they only ever match at the very start and end of the subject string, respectively, whatever options are set. When the PCRE_EXTRA flag is set on a call to pcre_compile(), the additional assertion \X, which has no equivalent in Perl, is recognized. This operates like the "cut" operation in Prolog: it prevents the matching operation from backtracking past it. For example, if the expression
  .*/foo is matched against the string "/this/string/is/not" then after the greedy .* has swallowed the whole string, PCRE keeps backtracking all the way to the beginning before failing. If, on the other hand, the expression is
  .*/\Xfoo then once it has discovered that "/not" is not "/foo", backtracking ceases, and the match fails. See also the section on "once-only" subpatterns below.

CIRCUMFLEX AND DOLLAR
Outside a character class, the circumflex character is an assertion which is true only if the current matching point is at the start of the subject string, in the default matching mode. 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. The meanings of the circumflex and dollar characters are changed if the PCRE_MULTILINE option is set at compile or matching time. 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. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. Note that the sequences "\A" 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.

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, then 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 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 \. 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. 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 \ 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 character "]" as the end character of a range, since a sequence such as [w-] is interpreted as a class of two characters. The octal or hexadecimal representation of "]" can, however, 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 such as [W-c] is used when PCRE_CASELESS is set, it matches the letters involved in either case. 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, the class [^\W_] matches any letter or digit. 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.

VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. The matching process tries all the alternatives in turn. For example, the pattern
  gilbert|sullivan matches either "gilbert" or "sullivan". Any number of alternatives can be used, and an empty alternative is permitted (matching the empty string).

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.

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 if there have not been that many previous capturing left parentheses. 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". There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, then 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, then 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.

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
  a parenthesized subpattern 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} 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, then 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) then the quantifiers are not greedy by default, but individual ones can be made greedy by following they by 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 .* then it is implicitly anchored, since whatever follows will be tried against every character position in the subject string. PCRE treats this as though it were preceded by \A. When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration. For example,
   (*tweedle[dume]{3})+\1 matches "tweedledum tweedledee tweedledee" but not "tweedledum tweedledee tweedledum".

ASSERTIONS
An assertion is a test on the characters following the current matching point that does not actually consume any of those characters. The simple assertions coded as \b, \B, \A, \Z, ^ and $ are described above. More complicated assertions are coded as subpatterns starting 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". Assertion subpatterns are not capturing subpatterns, and may not be repeated, because it makes no sense to assert the same thing several times. If an assertion contains capturing subpatterns within it, these are always counted for the purposes of numbering the capturing subpatterns in the whole pattern. Substring capturing is carried out for positive assertions, but it does not make sense for negative assertions. Assertions count towards the maximum of 200 parenthesized subpatterns.

ONCE-ONLY SUBPATTERNS
The facility described in this section is available only when the PCRE_EXTRA option is set at compile time. It is an extension to Perl regular expressions. 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:
  (?>+)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. For simple cases such as the above example, this feature can be though 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. Contrast with the \X assertion, which is a Prolog-like "cut".

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.

INTERNAL FLAG SETTING
If the sequence (?i) occurs anywhere in a pattern, it has the effect of setting the PCRE_CASELESS option, that is, all letters are matched in a case-independent manner. The option applies to the whole pattern, not just to the portion that follows it. If the sequence (?m) occurs anywhere in a pattern, it has the effect of setting the PCRE_MULTILINE option, that is, subject strings matched by this pattern are treated as consisting of multiple lines. If the sequence (?s) occurs anywhere in a pattern, it has the effect of setting the PCRE_DOTALL option, so that dot metacharacters match newlines as well as all other characters. If the sequence (?x) occurs anywhere in a pattern, it has the effect of setting the PCRE_EXTENDED option, that is, whitespace is ignored and # introduces a comment that lasts till the next newline. The option applies to the whole pattern, not just to the portion that follows it. If the sequence (?U) occurs anywhere in a pattern, it has the effect of setting the PCRE_UNGREEDY option which inverts the greediness of quantifiers. This is an extension to Perl's facilities. If the sequence (?X) occurs in a pattern, it has the effect of setting the PCRE_EXTRA flag, which turns on some additional features not found in Perl. This flag setting is special in that it must occur earlier in the pattern than any of the additional features. It is best put at the start. If more than one option is required, they can be specified jointly, for example as (?ix) or (?mi).

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. The use of PCRE_MULTILINE causes additional processing and should be avoided when it is not necessary. Caseless matching of character classes is more efficient if PCRE_CASELESS is set when the pattern is compiled.

AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714 Copyright (c) 1998 University of Cambridge.