grep: Back-references and Subexpressions

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1 3.5 Back-references and Subexpressions
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1 The back-reference ‘\N’, where N is a single digit, matches the
1 substring previously matched by the Nth parenthesized subexpression of
1 the regular expression.  For example, ‘(a)\1’ matches ‘aa’.  When used
1 with alternation, if the group does not participate in the match then
1 the back-reference makes the whole match fail.  For example, ‘a(.)|b\1’
1 will not match ‘ba’.  When multiple regular expressions are given with
1 ‘-e’ or from a file (‘-f FILE’), back-references are local to each
1 expression.
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