REGEX(7)							Linux Programmer's Manual							 REGEX(7)


NAME
       regex - POSIX.2 regular expressions

DESCRIPTION
       Regular expressions ("RE"s), as defined in POSIX.2, come in two forms: modern REs (roughly those of egrep; POSIX.2 calls these "extended" REs) and
       obsolete REs (roughly those of ed(1); POSIX.2 "basic" REs).  Obsolete REs mostly exist for backward compatibility in some old programs; they  will
       be  discussed  at  the  end.   POSIX.2 leaves some aspects of RE syntax and semantics open; "(!)" marks decisions on these aspects that may not be
       fully portable to other POSIX.2 implementations.

       A (modern) RE is one(!) or more nonempty(!) branches, separated by '|'.	It matches anything that matches one of the branches.

       A branch is one(!) or more pieces, concatenated.	 It matches a match for the first, followed by a match for the second, etc.

       A piece is an atom possibly followed by a single(!) '*', '+', '?', or bound.  An atom followed by '*' matches a sequence of 0 or more  matches  of
       the  atom.   An	atom  followed	by '+' matches a sequence of 1 or more matches of the atom.  An atom followed by '?' matches a sequence of 0 or 1
       matches of the atom.

       A bound is '{' followed by an unsigned decimal integer, possibly followed by ',' possibly followed by another  unsigned	decimal	 integer,  always
       followed	 by  '}'.   The integers must lie between 0 and RE_DUP_MAX (255(!)) inclusive, and if there are two of them, the first may not exceed the
       second.	An atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom.	An atom	 followed
       by  a  bound containing one integer i and a comma matches a sequence of i or more matches of the atom.  An atom followed by a bound containing two
       integers i and j matches a sequence of i through j (inclusive) matches of the atom.

       An atom is a regular expression enclosed in "()" (matching a match for  the  regular  expression),  an  empty  set  of  "()"  (matching	the  null
       string)(!),  a bracket expression (see below), '.' (matching any single character), '^' (matching the null string at the beginning of a line), '$'
       (matching the null string at the end of a line), a '\' followed by one of the characters "^.[$()|*+?{\" (matching that character taken as an ordi-
       nary  character),  a  '\'  followed by any other character(!)  (matching that character taken as an ordinary character, as if the '\' had not been
       present(!)), or a single character with no other significance (matching that character).	 A '{' followed by a character other than a digit  is  an
       ordinary character, not the beginning of a bound(!).  It is illegal to end an RE with '\'.

       A  bracket  expression  is a list of characters enclosed in "[]".  It normally matches any single character from the list (but see below).  If the
       list begins with '^', it matches any single character (but see below) not from the rest of the list.  If two characters in the list are	separated
       by  '-', this is shorthand for the full range of characters between those two (inclusive) in the collating sequence, for example, "[0-9]" in ASCII
       matches any decimal digit.  It is illegal(!) for two ranges to share an endpoint, for example, "a-c-e".	Ranges are very collating-sequence-depen-
       dent, and portable programs should avoid relying on them.

       To include a literal ']' in the list, make it the first character (following a possible '^').  To include a literal '-', make it the first or last
       character, or the second endpoint of a range.  To use a literal '-' as the first endpoint of a range, enclose it in "[." and ".]"  to  make  it	a
       collating  element  (see below).	 With the exception of these and some combinations using '[' (see next paragraphs), all other special characters,
       including '\', lose their special significance within a bracket expression.

       Within a bracket expression, a collating element (a character, a multicharacter sequence that collates as if it were a single character, or a col-
       lating-sequence	name  for  either)  enclosed in "[." and ".]" stands for the sequence of characters of that collating element.	The sequence is a
       single element of the bracket expression's list.	 A bracket expression containing a multicharacter collating element can thus match more than  one
       character, for example, if the collating sequence includes a "ch" collating element, then the RE "[[.ch.]]*c" matches the first five characters of
       "chchcc".

       Within a bracket expression, a collating element enclosed in "[=" and "=]" is an equivalence class, standing for the sequences  of  characters  of
       all collating elements equivalent to that one, including itself.	 (If there are no other equivalent collating elements, the treatment is as if the
       enclosing delimiters were "[." and ".]".)  For example, if o and ^ are the members of an equivalence class, then "[[=o=]]", "[[=^=]]", and  "[o^]"
       are all synonymous.  An equivalence class may not(!) be an endpoint of a range.

       Within  a  bracket  expression,	the  name  of a character class enclosed in "[:" and ":]" stands for the list of all characters belonging to that
       class.  Standard character class names are:

	      alnum   digit   punct
	      alpha   graph   space
	      blank   lower   upper
	      cntrl   print   xdigit

       These stand for the character classes defined in wctype(3).  A locale may provide others.  A character class may not be used as an endpoint  of	a
       range.

       In  the event that an RE could match more than one substring of a given string, the RE matches the one starting earliest in the string.	If the RE
       could match more than one substring starting at that point, it matches the longest.  Subexpressions also match the  longest  possible  substrings,
       subject	to  the	 constraint that the whole match be as long as possible, with subexpressions starting earlier in the RE taking priority over ones
       starting later.	Note that higher-level subexpressions thus take priority over their lower-level component subexpressions.

       Match lengths are measured in characters, not collating elements.  A null string is considered longer than no match at all.   For  example,  "bb*"
       matches	the three middle characters of "abbbc", "(wee|week)(knights|nights)" matches all ten characters of "weeknights", when "(.*).*" is matched
       against "abc" the parenthesized subexpression matches all three characters, and when "(a*)*" is matched against "bc" both the  whole  RE	 and  the
       parenthesized subexpression match the null string.

       If case-independent matching is specified, the effect is much as if all case distinctions had vanished from the alphabet.  When an alphabetic that
       exists in multiple cases appears as an ordinary character outside a bracket expression, it is effectively transformed into  a  bracket  expression
       containing both cases, for example, 'x' becomes "[xX]".	When it appears inside a bracket expression, all case counterparts of it are added to the
       bracket expression, so that, for example, "[x]" becomes "[xX]" and "[^x]" becomes "[^xX]".

       No particular limit is imposed on the length of REs(!).	Programs intended to be portable should not employ REs	longer	than  256  bytes,  as  an
       implementation can refuse to accept such REs and remain POSIX-compliant.

       Obsolete ("basic") regular expressions differ in several respects.  '|', '+', and '?' are ordinary characters and there is no equivalent for their
       functionality.  The delimiters for bounds are "\{" and "\}", with '{' and '}' by themselves ordinary characters.	 The parentheses for nested  sub-
       expressions are "\(" and "\)", with '(' and ')' by themselves ordinary characters.  '^' is an ordinary character except at the beginning of the RE
       or(!) the beginning of a parenthesized subexpression, '$' is an ordinary character except at the end of the RE or(!) the end  of	 a  parenthesized
       subexpression,  and '*' is an ordinary character if it appears at the beginning of the RE or the beginning of a parenthesized subexpression (after
       a possible leading '^').

       Finally, there is one new type of atom, a back reference: '\' followed by a nonzero decimal digit  d  matches  the  same	 sequence  of  characters
       matched	by  the dth parenthesized subexpression (numbering subexpressions by the positions of their opening parentheses, left to right), so that,
       for example, "\([bc]\)\1" matches "bb" or "cc" but not "bc".

BUGS
       Having two kinds of REs is a botch.

       The current POSIX.2 spec says that ')' is an ordinary character in the absence of an unmatched '('; this was an unintentional result of a  wording
       error, and change is likely.  Avoid relying on it.

       Back  references	 are  a	 dreadful  botch,  posing  major  problems  for	 efficient implementations.  They are also somewhat vaguely defined (does
       "a\(\(b\)*\2\)*d" match "abbbd"?).  Avoid using them.

       POSIX.2's specification of case-independent matching is vague.  The "one case implies all cases" definition given above is current consensus among
       implementors as to the right interpretation.

AUTHOR
       This page was taken from Henry Spencer's regex package.

SEE ALSO
       grep(1), regex(3)

       POSIX.2, section 2.8 (Regular Expression Notation).

COLOPHON
       This  page  is  part  of	 release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can be
       found at http://www.kernel.org/doc/man-pages/.



									2009-01-12								 REGEX(7)