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RE_FORMAT(7)							  RE_FORMAT(7)

NAME
       re_format - 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; 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 implemen‐
       tations.

       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  fol‐
       lowed  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† (match‐
       ing 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 nor‐
       mally  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  charac‐
       ters  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 (fol‐
       lowing 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 spe‐
       cial 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  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	 ctype(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 char‐
       acters.	A  word	 character  is	an  alnum  character  (as  defined  by
       ctype(3))  or an underscore.  This is an extension, compatible with but
       not specified by POSIX 1003.2, and should be used with caution in soft‐
       ware 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 pri‐
       ority  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	`week‐
       nights', when `(.*).*' is matched against `abc' the parenthesized  sub‐
       expression  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,  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-com‐
       pliant.

       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 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	 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	 effi‐
       cient  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 con‐
       sensus among implementors as to the right interpretation.

       The syntax for word boundaries is incredibly ugly.

				March 16, 1994			  RE_FORMAT(7)
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