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RedHat 9 (Linux i386) - man page for regex (redhat section 7)

REGEX(7)										 REGEX(7)

NAME
       regex - POSIX 1003.2 regular expressions

DESCRIPTION
       Regular	expressions  (``RE''s), as defined in POSIX 1003.2, come in two forms: modern REs
       (roughly those of egrep; 1003.2 calls these ``extended'' REs) and  obsolete  REs  (roughly
       those of ed(1); 1003.2 ``basic'' REs).  Obsolete REs mostly exist for backward compatibil-
       ity in some old programs; they will be discussed at the end.  1003.2 leaves  some  aspects
       of  RE  syntax  and semantics open; `(!)' marks decisions on these aspects that may not be
       fully portable to other 1003.2 implementations.

       A (modern) RE is one(!) or more non-empty(!) 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, e.g. `[0-9]' in ASCII matches any decimal
       digit.  It is illegal(!) for two ranges to share an endpoint, e.g.  `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 multi-character  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 multi-character collating element can thus match  more  than  one  character,
       e.g. 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.

       There are two special cases(!) of bracket expressions: the bracket  expressions	`[[:<:]]'
       and  `[[:>:]]'  match  the null string at the beginning and end of a word respectively.	A
       word is defined as a sequence of word characters which is neither preceded nor followed by
       word  characters.   A word character is an alnum character (as defined by wctype(3)) or an
       underscore.  This is an extension, compatible with but not specified by POSIX 1003.2,  and
       should be used with caution in software intended to be portable to other systems.

       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, e.g. `x' becomes `[xX]'.  When it appears
       inside a bracket expression, all case counterparts of it are added to the bracket  expres-
       sion, so that (e.g.) `[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  delim-
       iters  for  bounds  are `\{' and `\}', with `{' and `}' by themselves ordinary characters.
       The parentheses for nested subexpressions are `\(' and `\)', with `('  and  `)'	by  them-
       selves  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  paren-
       thesized  subexpression (after a possible leading `^').	Finally, there is one new type of
       atom, a back reference: `\' followed by a  non-zero  decimal  digit  d  matches	the  same
       sequence  of  characters  matched by the dth parenthesized subexpression (numbering subex-
       pressions by the positions of their opening parentheses, left to right),  so  that  (e.g.)
       `\([bc]\)\1' matches `bb' or `cc' but not `bc'.

SEE ALSO
       regex(3)

       POSIX 1003.2, section 2.8 (Regular Expression Notation).

BUGS
       Having two kinds of REs is a botch.

       The  current  1003.2  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.

       1003.2'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.

       The syntax for word boundaries is incredibly ugly.

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

					    1994-02-07					 REGEX(7)


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