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3.5 Back-references and Subexpressions

The back-reference ‘\n’, where n is a single nonzero digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression. For example, ‘(a)\1’ matches ‘aa’. If the parenthesized subexpression does not participate in the match, the back-reference makes the whole match fail; for example, ‘(a)*\1’ fails to match ‘a’. If the parenthesized subexpression matches more than one substring, the back-reference refers to the last matched substring; for example, ‘^(ab*)*\1$’ matches ‘ababbabb’ but not ‘ababbab’. When multiple regular expressions are given with -e or from a file (‘-f file’), back-references are local to each expression.

See Known Bugs, for some known problems with back-references.