Please read the forum post guidelines before posting next time. There is very little information given here. I will assume a few things here. Firts that you only want to blank out that line and not the rest of the file. With this sample data

1 omfg woot oh yeah
2 w00t yeah
3 omfg blah blah
4 w00t yeah
5 w00t yeah
6 w00t yeah
7 omfg gone gone gone
8 w00t yeah
9 omfg weeeeeeee!!!!

I will use this regex

(?<=omfg).*

It will match all the text that comes after "omfg". So if you replace that with an empty string. 

1 omfg
2 w00t yeah
3 omfg
4 w00t yeah
5 w00t yeah
6 w00t yeah
7 omfg
8 w00t yeah
9 omfg

 I hope that helps, and please read the posting guidelines next time.

Brendan