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
     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 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 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

     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
     o'o^'   are   the  members  of  an  equivalence  class,  then  `[[=o=]]',
     `[[=o'o^'=]]', and `[oo'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

     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 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 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

     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-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'.


     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  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.