Using Substrings, you can reuse parts of the expression to match for the replacement. This is a simple example:

$ date
Mon Jul 25 23:59:40 CEST 2005

$ date | sed -n 's/.*\ \(.*\)/\1/p'
2005

Important parts are:

Strangely, the order of evaluation of Regular Expressions works from right to left. The simple example below proofs this:

$ release=`uname -r`

$ echo $release
2.6.12-gentoo-r6

$ expr $release : '\(.*\)\..*'
2.6

$ expr $release : '\(.*\)\..*\..*'
2

$ expr $release : '\(.*\..*\)\..*'
2.6

$ expr $release : '\(.*\..*\).*'
2.6.12-gentoo-r6

Clearly to be seen, the “.*” right of the brackets has higher priority than the one inside the brackets.