NAME

regexp − regular expression notation

DESCRIPTION

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. 

SEE ALSO

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

Plan 9  —  March 26, 2000