Another pattern that
requires a special mechanism is searching for repeated words. The expression
[a-z][a-z] will match any two lowercase
letters. If you wanted to search for lines that had two adjoining identical
letters, the above pattern wouldn't help. You need a way to remember what you
found and see if the same pattern occurs again. In some programs, you can mark
part of a pattern using \( and \). You can recall the remembered pattern with
\ followed by a single digit.[4] Therefore, to search for two identical letters, use \([a-z]\)\1. You can have nine different
remembered patterns. Each occurrence of \(
starts a new pattern. The regular expression to match a five-letter palindrome
(e.g., "radar") is: \([a-z]\)\([a-z]\)[a-z]\2\1. [Some versions of some programs
can't handle \( \) in the same regular
expression as \1, etc. In all versions of
sed, you're safe if you use \( \) on the pattern side of an
s command — and \1,
etc., on the replacement side (Section
34.11). — JP]
— BB
[4] In Perl, you can also use $1
through $9 and even beyond, with the
right switches, in addition to the backslash mechanism.