REGEX(7)						 Miscellaneous Information Manual						  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 compatibility 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 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 ordinary 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  charac-
       ter 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 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 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 equivalence class, standing for the sequences  of  charac-
       ters  of all collating elements equivalent to that one, including itself.  (If there are no other equivalent collating elements, the treat-
       ment 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 begin-
       ning 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 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 subexpres-
       sions.

       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 alpha-
       betic 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 expression, 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 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 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 non-zero 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 (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 consen-
       sus 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)