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PCRE(7)

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NAME    [Toc]    [Back]

       pcre - Perl-compatible regular expressions: expresion syntax.


REGULAR EXPRESSION DETAILS    [Toc]    [Back]

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

       The  description here is intended as reference documentation. 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 characters in the  subject.  As  a
       trivial example, the pattern

	 The quick brown fox

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

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

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

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

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

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


BACKSLASH    [Toc]    [Back]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


CIRCUMFLEX AND DOLLAR    [Toc]    [Back]

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

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

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

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

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

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



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


SQUARE BRACKETS    [Toc]    [Back]

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

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

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

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

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

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

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

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

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

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


POSIX CHARACTER CLASSES    [Toc]    [Back]

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

	 [01[:alpha:]%]

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

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

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

	 [12[:^digit:]]

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


VERTICAL BAR    [Toc]    [Back]

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

	 gilbert|sullivan

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



INTERNAL OPTION SETTING    [Toc]    [Back]

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

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

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

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

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

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

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

	 (a(?i)b)c

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

	 (a(?i)b|c)

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

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


SUBPATTERNS    [Toc]    [Back]

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

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

	 cat(aract|erpillar|)

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

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

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


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

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

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

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

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

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

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

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


REPETITION    [Toc]    [Back]

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

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

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

	 z{2,4}

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

	 [aeiou]{3,}

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

	 \d{8}

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

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

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

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

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

	 (a?)*

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


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

	 /\*.*\*/

       to the string

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

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

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

	 /\*.*?\*/

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

	 \d??\d

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

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

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

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

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

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

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

	 /(a|(b))+/

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


BACK REFERENCES    [Toc]    [Back]

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

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

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

	 (sens|respons)e and \1ibility

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


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

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

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

	 (a|(bc))\2

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

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

	 (a|b\1)+

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


ASSERTIONS    [Toc]    [Back]

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

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

	 \w+(?=;)

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

	 foo(?!bar)

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

	 (?!foo)bar

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

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

	 (?<!foo)bar

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

	 (?<=bullock|donkey)

       is permitted, but

	 (?<!dogs?|cats?)

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

	 (?<=ab(c|de))

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

	 (?<=abc|abde)

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

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

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

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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

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

       Assertions count towards the maximum of 200 parenthesized  subpatterns.


ONCE-ONLY SUBPATTERNS    [Toc]    [Back]

       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. Onceonly
 subpatterns provide the means for specifying that once  a  portion
       of  the	pattern has matched, it is not to be re-evaluated in this way,
       so the matcher would give up immediately on failing to match "foo"  the
       first time. The notation is another kind of special parenthesis, starting
 with (?> as in this example:

	 (?>\d+)bar

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

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

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

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

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

	 abcd$

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

	 ^.*abcd$

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

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

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

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

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

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

	 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

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

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

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


CONDITIONAL SUBPATTERNS    [Toc]    [Back]

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

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

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

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

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

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

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

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

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


COMMENTS    [Toc]    [Back]

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

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


RECURSIVE PATTERNS    [Toc]    [Back]

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

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

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

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

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

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

	 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

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

	 (ab(cd)ef)

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

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


PERFORMANCE    [Toc]    [Back]

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

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

	 (.*) second

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

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

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

	 (a+)*

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

       An optimization catches some of the more simple cases such as

	 (a+)*b

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

	 (a+)*\d

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


UTF-8 SUPPORT    [Toc]    [Back]

       Starting  at  release  3.3, PCRE has some support for character strings
       encoded in the UTF-8 format. This is incomplete,  and  is  regarded  as
       experimental.  In  order  to use it, you must configure PCRE to include
       UTF-8 support in the code, and, in addition, you  must  call  pcre_com-
       pile()  with the PCRE_UTF8 option flag. When you do this, both the pattern
 and any subject strings that are matched against it are treated as
       UTF-8  strings  instead of just strings of bytes, but only in the cases
       that are mentioned below.

       If you compile PCRE with UTF-8 support, but do not use it at run  time,
       the  library will be a bit bigger, but the additional run time overhead
       is limited to testing the PCRE_UTF8 flag in several places,  so	should
       not be very large.

       PCRE assumes that the strings it is given contain valid UTF-8 codes. It
       does not diagnose invalid UTF-8 strings.  If  you  pass	invalid  UTF-8
       strings to PCRE, the results are undefined.

       Running with PCRE_UTF8 set causes these changes in the way PCRE works:

       1. In a pattern, the escape sequence \x{...}, where the contents of the
       braces is a string of hexadecimal digits, is  interpreted  as  a  UTF-8
       character  whose code number is the given hexadecimal number, for example:
 \x{1234}. This inserts from one to six literal bytes into the pattern,
  using  the  UTF-8  encoding.  If a non-hexadecimal digit appears
       between the braces, the item is not recognized.

       2. The original hexadecimal escape sequence, \xhh, generates a two-byte
       UTF-8 character if its value is greater than 127.

       3. Repeat quantifiers are NOT correctly handled if they follow a multibyte
 character. For example, \x{100}* and \xc3+ do  not	work.  If  you
       want  to repeat such characters, you must enclose them in non-capturing
       parentheses, for example (?:\x{100}), at present.

       4. The dot metacharacter matches one UTF-8 character instead of a  single
 byte.

       5. Unlike literal UTF-8 characters, the dot metacharacter followed by a
       repeat quantifier does operate correctly on UTF-8 characters instead of
       single bytes.

       4. Although the \x{...} escape is permitted in a character class, characters
 whose values are greater than 255 cannot be included in a class.

       5.  A class is matched against a UTF-8 character instead of just a single
 byte, but it can match only characters whose values are  less  than
       256. Characters with greater values always fail to match a class.

       6. Repeated classes work correctly on multiple characters.

       7.  Classes  containing	just a single character whose value is greater
       than 127 (but less than 256), for example, [\x80] or [^\x{93}], do  not
       work because these are optimized into single byte matches. In the first
       case, of course, the class brackets are just redundant.

       8. Lookbehind assertions move backwards in the subject by a fixed  number
 of characters instead of a fixed number of bytes. Simple cases have
       been tested to work correctly, but there may be hidden gotchas  herein.

       9.  The	character  types  such as \d and \w do not work correctly with
       UTF-8 characters. They continue to test a single byte.

       10. Anything not explicitly mentioned here continues to work  in  bytes
       rather than in characters.


DIFFERENCES FROM PERL    [Toc]    [Back]

       The differences described here are with respect to Perl 5.005.

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

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

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

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

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

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

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

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

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

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

       10. The following UTF-8 features of Perl 5.6 are not implemented:

       a. The escape sequence \C to match a single byte.

       b. The use of Unicode tables and properties and escapes \p, \P, and \X.

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


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

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

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

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

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

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

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


AUTHOR    [Toc]    [Back]

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

  

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