REGEXP(6)                                               REGEXP(6)

     NAME
          regexp - regular expression notation

     DESCRIPTION
          A regular expression specifies a set of strings of charac-
          ters.  A member of this set of strings is said to be matched
          by the regular expression.  In many applications a delimiter
          character, commonly `/', bounds a regular expression.  In
          the following specification for regular expressions the word
          `character' means any character (rune) but newline.

          The syntax for a regular expression e0 is

               e3:  literal | charclass | '.' | '^' | '$' | '(' e0 ')'

               e2:  e3
                 |  e2 REP

               REP: '*' | '+' | '?'

               e1:  e2
                 |  e1 e2

               e0:  e1
                 |  e0 '|' e1

          A literal is any non-metacharacter, or a metacharacter (one
          of .*+?[]()|\^$), or the delimiter preceded by `\'.

          A charclass is a nonempty string s bracketed [s] (or [^s]);
          it matches any character in (or not in) s. A negated charac-
          ter class never matches newline.  A substring a-b, with a
          and b in ascending order, stands for the inclusive range of
          characters between a and b. In s, the metacharacters `-',
          `]', an initial `^', and the regular expression delimiter
          must be preceded by a `\'; other metacharacters have no spe-
          cial meaning and may appear unescaped.

          A `.'  matches any character.

          A `^' matches the beginning of a line; `$' matches the end
          of the line.

          The REP operators match zero or more (*), one or more (+),
          zero or one (?), instances respectively of the preceding
          regular expression e2.

          A concatenated regular expression, e1e2, matches a match to
          e1 followed by a match to e2.

     REGEXP(6)                                               REGEXP(6)

          An alternative regular expression, e0|e1, matches either a
          match to e0 or a match to e1.

          A match to any part of a regular expression extends as far
          as possible without preventing a match to the remainder of
          the regular expression.

     SEE ALSO
          awk(1), ed(1), grep(1), sam(1), sed(1), regexp(2)