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 character 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 special 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.

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.