
RE FORMAT(7)                System Reference Manual               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(1);  1003.2 calls these ``ex-
     tended'' 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 fol-
     lowed by `}'.  The integers must lie between 0 and RE DUP MAX (255) in-
     clusive, and if there are two of them, the first may not exceed the sec-
     ond.  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 in-
     tegers 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), `$' (matching
     the null string at the end of a line), a `\' followed by one of the char-
     acters `^.[$()|*+?{\' (matching that character taken as an ordinary char-
     acter), 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 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 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 (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 special charac-
     ters, including `\', lose their special significance within a bracket ex-
     pression.

     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
     are the members of an equivalence class, then `[[=o=]]', `[[=o=]]', and
     `[oo]' 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 expres-
     sions `[[:<:]]' 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 un-
     derscore.  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 expres-
     sion 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]' be-
     comes `[^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 imple-
     mentation 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 nest-
     ed subexpressions are `\(' and `\)', with `(' and `)' by themselves ordi-
     nary characters.  `^' is an ordinary character except at the beginning of
     the RE or the beginning of a parenthesized subexpression, `$' is an ordi-
     nary 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 be-
     ginning of the RE or the beginning of a parenthesized subexpression (af-
     ter 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 subexpres-
     sion (numbering subexpressions by the positions of their opening paren-
     theses, 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 ab-
     sence 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.

BSD Experimental                March 20, 1994                               3
