I use and love the text editor Textmate, which has some powerful functions. One thing that it can help with is quickly editing text files, and for example today I used it for searching lines in a mail system's "aliases" file. I wanted to remove 50-odd lines with the word owner in them, so I used the Find command with Regular Expression checked.
The search string is:
If you enter that string which means to find the lines with owner in them, check "Regular Expression," and leave a blank in the Replace box, Textmate will blank out the lines for you. Convenient!