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Manual Reference Pages  -  REGEXP (7)


regexp - Plan 9 regular expression notation




This manual page describes the regular expression syntax used by the Plan 9 regular expression library regexp(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 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.


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