Regular Expression Syntax

       re_format - POSIX 1003.2 regular expressions

       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 pro­
       grams; 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, sepa­
       rated by `|'.  It matches anything that matches one of the

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

       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 fol­
       lowed 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-) inclu­
       sive, 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 contain­
       ing 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), `^'  (match­
       ing  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 `\' fol­
       lowed  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 signifi­
       cance (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 sepa­
       rated 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  lit­
       eral `-', make it the first or last character, or the sec­
       ond 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  excep­
       tion  of  these	and some combinations using `[' (see next
       paragraphs), all other special characters, including  `\',
       lose  their  special significance within a bracket expres­

       Within a bracket expression, a collating element (a  char­
       acter,  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  equiva­
       lent  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 syn­
       onymous.  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 char­
       acters  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 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 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 software intended to be portable to other  sys­

       In  the	event  that  an RE could match more than one sub­
       string 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 ear­
       lier 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, 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

       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.   `|'	is  an ordinary character and there is no
       equivalent for its functionality.  `+' and `?'  are  ordi­
       nary  characters, and their functionality can be expressed
       using bounds ({1,} or {0,1} respectively).  Also note that
       `x+' in modern REs is equivalent to `xx*'.  The delimiters
       for bounds are `\{' and `\}', with `{' and  `}'	by  them­
       selves  ordinary  characters.   The parentheses for nested
       subexpressions are `\(' and `\)',  with	`('  and  `)'  by
       themselves  ordinary characters.  `^' is an ordinary char­
       acter except at the beginning of the RE or- the	beginning
       of a parenthesized subexpression, `$' is an ordinary char­
       acter except at the end of the RE or- the end of a  paren­
       thesized  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 ref­
       erence: `\' followed by a non-zero decimal digit d matches
       the same sequence of characters matched by the dth  paren­
       thesized  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


       POSIX 1003.2, section 2.8 (Regular Expression Notation).

       Having two kinds of REs is a botch.

       The current 1003.2 spec says that `)' is an ordinary char­
       acter  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 prob­
       lems for efficient implementations.  They are  also  some­
       what   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.