Reference A Capture In The Regex

You create a capture group in a sed regex by wrapping a pattern in \( and \). Generally, this capture group is referenced in the substitution expression with \1.

The capture references (e.g. \1) can also be used in the regex as part of specifying the match.

For instance, we can do a capture of a single digit followed by a reference to that capture. That will match any line that has a pair of matching consecutive digits.

$ seq 111 | sed -n 's/\([[:digit:]]\)\1/&/p'
11
22
33
44
55
66
77
88
99
100
110
111

This also uses & in the subex which represents the entire match. The -n and /p combination suppresses printing of lines to only those that have substitutions.