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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  any-
       thing that matches one of the branches.

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

       A piece is an atom possibly followed by a single(!) '*', '+', '?', or bound.  An atom fol-
       lowed 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 expres-
       sion),  an  empty  set  of  "()"  (matching the null string)(!), a bracket expression (see
       below), '.' (matching any single character), '^' (matching the null string at  the  begin-
       ning  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 ordinary charac-
       ter), a '\' followed by any other character(!)  (matching that character taken as an ordi-
       nary 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-dependent,  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 end-
       point 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 collating-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 equiva-
       lence 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 sub-
       string  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 start-
       ing 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 con-
       sidered longer than no match at all.  For example, "bb*" matches the three middle  charac-
       ters  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 parenthe-
       sized 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 subexpressions 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 char-
       acter if it appears at the beginning of the RE or the beginning of a parenthesized  subex-
       pression (after a possible leading '^').

       Finally,  there is one new type of atom, a back reference: '\' followed by a nonzero deci-
       mal digit d matches the same sequence of characters matched by the dth parenthesized  sub-
       expression  (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 3.55 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)
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