String EDitor is a command line text editor that parses a file and makes changes one line at a time and as a consequence is very powerful. It can work with regular expressions to identify and replace text and is very quick.

Links

Sed One-Liners Explained, Part I: File Spacing, Numbering and Text Conversion and Substitution
Sed One-Liners Explained, Part II: Selective Printing of Certain Lines
Sed One-Liners Explained, Part III: Selective Deletion of Certain Lines and Special Applications