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+====== SED ======
+===== Substrings =====
+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'
+Important parts are:
+| **-n** | do not output anything not requested                             |
+| **\(** | marks the beginning of the Substring                              |
+| **\)** | marks the end of the Substring                                         |
+| **\1** | matches the first Substring defined in the expression |
+| **/p** | print the substitution                                                         |
+===== Order of Evaluation =====
+Strangely, the order of evaluation of Regular Expressions works from right to left. The simple example below proofs this:
+  $ release=`uname -r`
+  $ echo $release
+.6.12-gentoo-r6
+  $ expr $release : '\(.*\)\..*'
+.6
+  $ expr $release : '\(.*\)\..*\..*'
+  $ expr $release : '\(.*\..*\)\..*'
+.6
+  $ expr $release : '\(.*\..*\).*'
+.6.12-gentoo-r6
+Clearly to be seen, the ".*" right of the brackets has higher priority than the one inside the brackets.