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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       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 copi‐
       ous examples. Jeffrey Friedl's "Mastering  Regular  Expressions",  pub‐
       lished  by  O'Reilly, covers them in great detail. The description here
       is intended as reference documentation.

       The basic operation of PCRE is on strings of bytes. However,  there  is
       also  support for UTF-8 character strings. To use this support you must
       build PCRE to include UTF-8 support, and then call pcre_compile()  with
       the  PCRE_UTF8  option.	How  this affects the pattern matching is men‐
       tioned in several places below. There is also a summary of  UTF-8  fea‐
       tures in the section on UTF-8 support in the main pcre page.

       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 alterna‐
       tives 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	recog‐
       nized  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 string (or line, in multiline mode)
	 $	assert end of string (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
		also "possessive 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
	 [	POSIX character class (only if followed by POSIX
	 ]	terminates the character class

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


       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 escaping action 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 backslash 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 # charac‐
       ter as part of the pattern.

       If you want to remove the special meaning from a	 sequence  of  charac‐
       ters,  you can do so by putting them between \Q and \E. This is differ‐
       ent from Perl in that $ and  @  are  handled  as	 literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola‐
       tion. Note the following examples:

	 Pattern	    PCRE matches   Perl matches

	 \Qabc$xyz\E	    abc$xyz	   abc followed by the
					     contents of $xyz
	 \Qabc\$xyz\E	    abc\$xyz	   abc\$xyz
	 \Qabc\E\$\Qxyz\E   abc$xyz	   abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character

       A second use of backslash provides a way of encoding non-printing char‐
       acters 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)
	 \ddd	   character with octal code ddd, or backreference
	 \xhh	   character with hex code hh
	 \x{hhh..} character with hex code hhh... (UTF-8 mode only)

       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, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). In UTF-8 mode, any number of hexadecimal  dig‐
       its  may	 appear between \x{ and }, but the value of the character code
       must be less than 2**31 (that is,  the  maximum	hexadecimal  value  is
       7FFFFFFF).  If  characters other than hexadecimal digits appear between
       \x{ and }, or if there is no terminating }, this form of escape is  not
       recognized.  Instead,  the  initial  \x	will be interpreted as a basic
       hexadecimal escape, with no following digits, giving a byte whose value
       is zero.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x when PCRE is in UTF-8 mode. There is no  difference
       in  the	way they are handled. For example, \xdc is exactly the same as

       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  (code	 value	7).  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 compli‐
       cated.  Outside a character class, PCRE reads it and any following dig‐
       its 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  sin‐
       gle 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	might be a back reference, otherwise the
		   character with octal code 113
	 \377	might be a back reference, otherwise
		   the 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 or a single UTF-8
       character (in UTF-8 mode) 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.

       In  UTF-8 mode, characters with values greater than 255 never match \d,
       \s, or \w, and always match \D, \S, and \W.

       For compatibility with Perl, \s does not match the VT  character	 (code
       11).   This makes it different from the the POSIX "space" class. The \s
       characters are HT (9), LF (10), FF (12), CR (13), and space (32).

       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 defini‐
       tion of letters and digits is controlled by  PCRE's  character  tables,
       and  may vary if locale- specific matching is taking place (see "Locale
       support" in the pcreapi	page).	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 char‐
       acter  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 asser‐
       tion 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	matches at a word boundary
	 \B	matches when not at a word boundary
	 \A	matches at start of subject
	 \Z	matches at end of subject or before newline at end
	 \z	matches at end of subject
	 \G	matches at first matching position in subject

       These  assertions may not appear in character classes (but note that \b
       has a different meaning, namely the backspace character, inside a char‐
       acter 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.	 Thus,
       they are independent of multiline mode.

       They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
       startoffset argument of pcre_exec() is non-zero, indicating that match‐
       ing  is to start at a point other than the beginning of the subject, \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.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu‐
       ments, you can mimic Perl's /g option, and it is in this kind of imple‐
       mentation where \G can be useful.

       Note,  however,	that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of	the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one	 match
       at a time, it cannot reproduce this behaviour.

       If  all	the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


       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 argu‐
       ment of pcre_exec() is non-zero, circumflex  can	 never	match  if  the
       PCRE_MULTILINE  option  is  unset. 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  sub‐
       ject,  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 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 immedi‐
       ately after and	immediately  before  an	 internal  newline  character,
       respectively,  in addition to matching at the start and end of the sub‐
       ject 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 cir‐
       cumflex 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 it is always anchored, whether PCRE_MULTILINE is  set  or


       Outside a character class, a dot in the pattern matches any one charac‐
       ter in the subject, including a non-printing  character,	 but  not  (by
       default)	 newline.   In	UTF-8 mode, a dot matches any UTF-8 character,
       which might be more than one byte long, except (by  default)  for  new‐
       line.  If  the  PCRE_DOTALL option is set, dots match newlines as well.
       The handling of dot is entirely independent of the handling of  circum‐
       flex  and  dollar,  the	only relationship being that they both involve
       newline characters. Dot has no special meaning in a character class.


       Outside a character class, the escape sequence \C matches any one byte,
       both  in	 and out of UTF-8 mode. Unlike a dot, it always matches a new‐
       line. The feature is provided in Perl  in  order	 to  match  individual
       bytes  in UTF-8 mode.  Because it breaks up UTF-8 characters into indi‐
       vidual bytes, what remains in the  string  may  be  a  malformed	 UTF-8
       string. For this reason it is best avoided.

       PCRE  does not allow \C to appear in lookbehind assertions (see below),
       because in UTF-8 mode it makes it impossible to calculate the length of
       the lookbehind.


       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe‐
       cial. 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. In UTF-8
       mode, the character may occupy more than one byte. A matched  character
       must be in the set of characters defined by the class, unless the first
       character in the class definition 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.

       In UTF-8 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping

       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. PCRE does not support the
       concept of case for characters with values greater than 255.

       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  charac‐
       ters  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 charac‐
       ter 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 inter‐
       preted 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 the collating sequence of character values. They  can
       also   be  used	for  characters	 specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose	values
       are greater than 255, for example [\x{100}-\x{2ff}].

       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

       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.


       Perl  supports  the  POSIX  notation  for character classes, which uses
       names enclosed by [: and :] within the enclosing square brackets.  PCRE
       also supports this notation. For example,


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

	 alnum	  letters and digits
	 alpha	  letters
	 ascii	  character codes 0 - 127
	 blank	  space or tab only
	 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 (not quite the same as \s)
	 upper	  upper case letters
	 word	  "word" characters (same as \w)
	 xdigit	  hexadecimal digits

       The "space" characters are HT (9), LF (10), VT (11), FF (12), CR	 (13),
       and  space  (32). Notice that this list includes the VT character (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which	 is  indicated
       by a ^ character after the colon. For example,


       matches	"1", "2", or any non-digit. PCRE (and Perl) also recognize 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.

       In UTF-8 mode, characters with values greater than 255 do not match any
       of the POSIX character classes.


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


       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 alterna‐
       tives  are within a subpattern (defined below), "succeeds" means match‐
       ing the rest of the main pattern as well as the alternative in the sub‐


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

	 s  for PCRE_DOTALL

       For example, (?im) sets caseless, multiline matching. It is also possi‐
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE‐
       LESS  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.

       When  an option change occurs at top level (that is, not inside subpat‐
       tern parentheses), the change applies to the remainder of  the  pattern
       that follows.  If the change is placed right at the start of a pattern,
       PCRE extracts it into the global options (and it will therefore show up
       in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern affects only that part of the cur‐
       rent pattern that follows it, so


       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


       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 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


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

       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

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

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

       the captured substrings are "red king", "red", and "king", and are num‐
       bered 1, 2, and 3, respectively.

       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 a question mark and a colon, the subpattern does not do any  captur‐
       ing,  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 capturing subpatterns is 65535, and  the
       maximum	depth  of  nesting 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


       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


       Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the numbers in complicated regular expres‐
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.	To  help with the difficulty, PCRE supports the naming of sub‐
       patterns, something that Perl  does  not	 provide.  The	Python	syntax
       (?P<name>...)  is  used.	 Names	consist of alphanumeric characters and
       underscores, and must be unique within a pattern.

       Named capturing parentheses are still  allocated	 numbers  as  well  as
       names. The PCRE API provides function calls for extracting the name-to-
       number translation table from a compiled pattern. For  further  details
       see the pcreapi documentation.


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

	 a literal data character
	 the . metacharacter
	 the \C escape sequence
	 escapes such as \d that match single characters
	 a character class
	 a back reference (see next section)
	 a parenthesized subpattern (unless it is an assertion)

       The general repetition quantifier specifies a minimum and maximum  num‐
       ber  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:


       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


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


       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 exam‐
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char‐
       acters, each of which is represented by a two-byte sequence.

       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:


       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 bro‐

       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,	 indi‐
       vidual * 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 owing 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


       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 (equiv‐
       alent  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 normally treats such a pattern as though it were preceded by \A.

       In  cases  where	 it  is known that the subject string contains no new‐
       lines, it is worth setting PCRE_DOTALL in order to  obtain  this	 opti‐
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However,	 there is one situation where the optimization cannot be used.
       When .*	is inside capturing parentheses that  are  the	subject	 of  a
       backreference  elsewhere in the pattern, a match at the start may fail,
       and a later one succeed. Consider, for example:


       If the subject is "xyz123abc123" the match point is the fourth  charac‐
       ter. For this reason, such a pattern is not implicitly anchored.

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


       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	itera‐
       tions. For example, after


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


       With both maximizing and minimizing repetition, failure of what follows
       normally causes the repeated item to be re-evaluated to see if  a  dif‐
       ferent number of repeats allows the rest of the pattern to match. Some‐
       times 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


       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.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the	matcher	 would
       give up immediately on failing to match "foo" the first time. The nota‐
       tion is a kind of special parenthesis, starting with  (?>  as  in  this


       This  kind  of  parenthesis "locks up" the  part of the pattern it con‐
       tains 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.

       Atomic grouping 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  pre‐
       pared  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

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists	 of  an	 additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as


       Possessive  quantifiers	are  always  greedy;  the   setting   of   the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler forms of atomic group. However, there is no difference  in  the
       meaning	or  processing	of  a possessive quantifier and the equivalent
       atomic group.

       The possessive quantifier syntax is an extension to the Perl syntax. It
       originates in Sun's Java package.

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


       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


       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 opti‐
       mization 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


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


       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub‐
       pattern	earlier	 (that is, 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  pat‐
       tern.  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 dig‐
       its following a backslash.

       A back reference matches whatever actually matched the  capturing  sub‐
       pattern	in  the	 current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). 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 exam‐


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

       Back  references	 to named subpatterns use the Python syntax (?P=name).
       We could rewrite the above example as follows:


       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


       always fails if it starts to match "a" rather than "bc". Because	 there
       may  be	many  capturing parentheses in a pattern, 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 sub‐
       patterns. For example, the pattern


       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
       ation  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.


       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, \G, \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,


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


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


       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.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient	 way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an	 empty
       string must always fail.

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


       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 sev‐
       eral  alternatives, they do not all have to have the same fixed length.


       is permitted, but


       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	 (at  least  for  5.8),	 which
       requires	 all branches to match the same length of string. An assertion
       such as


       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:


       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 cur‐
       rent position, the match is deemed to fail.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode) to appear in lookbehind assertions, because it makes it  impossi‐
       ble to calculate the length of the lookbehind.

       Atomic  groups 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


       when  applied  to  a  long string that 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


       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


       or, equivalently,


       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.

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


       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" pre‐
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc‐
       foo". A pattern to do that is


       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,


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


       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  sub‐
       patterns in the whole pattern.  However, substring capturing is carried
       out only for positive assertions, because it does not  make  sense  for
       negative assertions.


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


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

       There are three 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. The number
       must be greater than zero. Consider the following pattern,  which  con‐
       tains  non-significant white space to make it more readable (assume the
       PCRE_EXTENDED option) and to divide it into three  parts	 for  ease  of

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

       The  first  part	 matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec‐
       ond  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-pat‐
       tern  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 the string (R), it is satisfied if a recursive call
       to  the pattern or subpattern has been made. At "top level", the condi‐
       tion is false.  This  is	 a  PCRE  extension.  Recursive	 patterns  are
       described in the next section.

       If  the	condition  is  not  a sequence of digits or (R), 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:

	 \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.


       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 new‐
       line character in the pattern.


       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  has provided an experimental facility that allows regular
       expressions to recurse (amongst other things). It does this by interpo‐
       lating  Perl code in the expression at run time, and the code can refer
       to the expression itself. A Perl pattern to solve the parentheses prob‐
       lem 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, it supports
       some special syntax for recursion of the entire pattern, and  also  for
       individual subpattern recursion.

       The  special item that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec‐
       tion.)  The special item (?R) is a recursive call of the entire regular

       For example, this PCRE pattern solves the  nested  parentheses  problem
       (assume	the  PCRE_EXTENDED  option  is	set  so	 that  white  space is

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

       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 (that is  a  correctly  parenthe‐
       sized substring).  Finally there is a closing parenthesis.

       If  this	 were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

	 ( \( ( (?>[^()]+) | (?1) )* \) )

       We have put the pattern into parentheses, and caused the	 recursion  to
       refer  to them instead of the whole pattern. In a larger pattern, keep‐
       ing track of parenthesis numbers can be tricky. It may be  more	conve‐
       nient  to use named parentheses instead. For this, PCRE uses (?P>name),
       which is an extension to the Python syntax that	PCRE  uses  for	 named
       parentheses (Perl does not provide named parentheses). We could rewrite
       the above example as follows:

	 (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )

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


       it  yields "no match" quickly. However, if atomic grouping 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.

       At the end of a match, the values set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value is set.  If you want to obtain  intermediate  values,  a  callout
       function	 can be used (see below and the pcrecallout documentation). If
       the pattern above is matched against


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

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

       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 pat‐
       tern, PCRE has to obtain extra memory to store data during a recursion,
       which  it  does	by  using pcre_malloc, freeing it via pcre_free after‐
       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the

       Do  not	confuse	 the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in	 angle	brack‐
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit‐
       ted at the outer level.

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

       In  this	 pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and	 non-recursive	cases.
       The (?R) item is the actual recursive call.


       If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it	 oper‐
       ates  like  a  subroutine in a programming language. An earlier example
       pointed out that the pattern

	 (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

	 (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Such references must, however, follow  the	subpattern  to
       which they refer.


       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub‐
       strings that match the same pair of parentheses when there is a repeti‐

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

       Within  a  regular  expression,	(?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout	points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout


       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the	number
       of  the	callout, and, optionally, one item of data originally supplied
       by the caller of pcre_exec(). The callout function may  cause  matching
       to  backtrack,  or  to  fail  altogether. A complete description of the
       interface to the callout function is given in the pcrecallout  documen‐

Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.


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