# 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 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
         `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
         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
         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 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
         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-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 consensus
         among implementors as to the right interpretation.

         The syntax for word boundaries is incredibly ugly.