====== 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'
  2005
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
  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.