Add Reference A Capture In The Regex as a sed til

sed/reference-a-capture-in-the-regex.md

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+# 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.
+
+```bash
+$ 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.