path: root/doc/pcre_man.txt

NAME
     pcre - Perl-compatible regular expressions.


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

     The description here is intended as reference documentation.
     The basic operation of PCRE is on strings of bytes. However,
     there is the beginnings of some support for UTF-8  character
     strings.  To  use  this  support  you must configure PCRE to
     include it, and then call pcre_compile() with the  PCRE_UTF8
     option.  How  this affects the pattern matching is described
     in the final section of this document.

     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 charac-
     ters 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  pat-
     tern.  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 follow-
     ing 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  particu-
     lar, if you want to match a backslash, you write "\\".

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

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

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

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

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

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

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

     Inside a character  class,  or  if  the  decimal  number  is
     greater  than  9 and there have not been that many capturing
     subpatterns, PCRE re-reads up to three octal digits  follow-
     ing  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  intro-
     duced  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 charac-
     ter types:

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

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

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

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

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

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

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

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

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



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

     Circumflex need not be the first character of the pattern if
     a  number of alternatives are involved, but it should be the
     first thing in each alternative in which it appears  if  the
     pattern is ever to match that branch. If all possible alter-
     natives 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 con-
     structs that can cause a pattern to be anchored.)

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

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

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

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



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



SQUARE BRACKETS
     An opening square bracket introduces a character class, ter-
     minated  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 cir-
     cumflex, if present) or escaped with a backslash.

     A character class matches a single character in the subject;
     the  character  must  be in the set of characters defined by
     the class, unless the first character in the class is a cir-
     cumflex,  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  con-
     venient  notation for specifying the characters which are in
     the class by enumerating those that are not. It  is  not  an
     assertion:  it  still  consumes a character from the subject
     string, and fails if the current pointer is at  the  end  of
     the string.

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

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

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

     It is not possible to have the literal character "]" as  the
     end  character  of  a  range.  A  pattern such as [W-]46] is
     interpreted as a class of two characters ("W" and "-")  fol-
     lowed 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 ASCII collating sequence. They can also be
     used  for  characters  specified  numerically,  for  example
     [\000-\037]. If a range that includes letters is  used  when
     caseless  matching  is set, it matches the letters in either
     case. For example, [W-c] is equivalent  to  [][\^_`wxyzabc],
     matched  caselessly,  and  if  character tables for the "fr"
     locale are in use, [\xc8-\xcb] matches accented E characters
     in both cases.

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

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



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

       [01[:alpha:]%]

     matches "0", "1", any alphabetic character, or "%". The sup-
     ported class names are

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

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

       [12[:^digit:]]

     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.



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

       gilbert|sullivan

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



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

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

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

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

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

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

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

       (a(?i)b)c

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

       (a(?i)b|c)

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

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



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

     1. It localizes a set of alternatives. For example, the pat-
     tern

       cat(aract|erpillar|)

     matches one of the words "cat",  "cataract",  or  "caterpil-
     lar".  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 por-
     tion 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 captur-
     ing 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, respectively.

     The fact that plain parentheses fulfil two functions is  not
     always  helpful.  There are often times when a grouping sub-
     pattern 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  pat-
     tern

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

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

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

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

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



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

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

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

       z{2,4}

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

       [aeiou]{3,}

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

       \d{8}

     matches exactly 8 digits.  An  opening  curly  bracket  that
     appears  in a position where a quantifier is not allowed, or
     one that does not match the syntax of a quantifier, is taken
     as  a literal character. For example, {,6} is not a quantif-
     ier, but a literal string of four characters.
     The quantifier {0} is permitted, causing the  expression  to
     behave  as  if the previous item and the quantifier were not
     present.

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

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

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

       (a?)*

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

     By default, the quantifiers  are  "greedy",  that  is,  they
     match  as much as possible (up to the maximum number of per-
     mitted 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  com-
     ments 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 pre-
     ferred number of matches.  Do not confuse this use of  ques-
     tion  mark  with  its  use as a quantifier in its own right.
     Because it has two uses, it can sometimes appear doubled, as
     in

       \d??\d

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

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

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

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

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

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

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

       /(a|(b))+/

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



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

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

     A back reference matches whatever actually matched the  cap-
     turing 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  responsi-
     bility",  but  not  "sense  and  responsibility". If caseful
     matching is in force at the time of the back reference,  the
     case of letters is relevant. For example,

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

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

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

       (a|(bc))\2

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

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

       (a|b\1)+

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



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

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

       \w+(?=;)

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

       foo(?!bar)

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

       (?!foo)bar

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

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

       (?<!foo)bar

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

       (?<=bullock|donkey)

     is permitted, but

       (?<!dogs?|cats?)

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

       (?<=ab(c|de))

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

       (?<=abc|abde)

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

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

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

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

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

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

     Assertions can be nested in any combination. For example,

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

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

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

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

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

     Assertions count towards the maximum  of  200  parenthesized
     subpatterns.



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

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

       123456bar

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

       (?>\d+)bar

     This kind of parenthesis "locks up" the  part of the pattern
     it  contains once it has matched, and a failure further into
     the pattern is prevented from backtracking  into  it.  Back-
     tracking  past  it to previous items, however, works as nor-
     mal.

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

     Once-only subpatterns are not capturing subpatterns.  Simple
     cases  such as the above example can be thought of as a max-
     imizing 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  compli-
     cated subpatterns, and it can be nested.

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

       abcd$

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

       ^.*abcd$

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

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

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

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

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

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

       aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

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

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

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



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

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

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

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

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

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

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

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

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



COMMENTS
     The sequence (?# marks the start of a comment which  contin-
     ues  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 contin-
     ues up to the next newline character in the pattern.



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

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

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

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

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

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

       (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

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

       (ab(cd)ef)

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

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



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

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

       (.*) second

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

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

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

       (a+)*

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

     An optimization catches some of the more simple  cases  such
     as

       (a+)*b

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

       (a+)*\d

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




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

     Last updated: 15 August 2001
     Copyright (c) 1997-2001 University of Cambridge.