regexp – Plan 9 regular expression notation

This manual page describes the regular expression syntax used by the Plan 9 regular expression library regexp9(3). It is the form used by egrep(1) before egrep got complicated.
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
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 ab, 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.