What does \1 in sed do?

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Here’s a simple example:

$ echo 'abcabcabc' | sed 's/\(ab\)c/\1/'
ababcabc
$ echo 'abcabcabc' | sed 's/\(ab\)c/\1/g'
ababab
$ echo 'abcabcabc' | sed 's/\(ab\)\(c\)/\1d\2/g'
abdcabdcabdc

In the first command, only the first match is affected. In the second command, every match is affected. In both cases, the \1 refers to the characters captured by the escaped parentheses.

In the third command, two capture groups are specified. They are referred to by using \1 and \2. Up to nine capture groups can be used.

In addition to the g (global) operator (or without it, the first match), you can specify a particular match:

$ echo 'aaaaaa' | sed 's/a/A/4'
aaaAaa

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