Regular Expressions: Remembering Patterns with \ (, \ ), and \1 (Unix Power Tools, 3rd Edition)

32.13. Regular Expressions: Remembering Patterns with \ (, \ ), and \1

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.[102] 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]

[102]In Perl, you can also use $1 through $9 and even beyond, with the right switches, in addition to the backslash mechanism.

-- BB