A regular expression specifies a set of strings of characters. 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
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.