Do be careful with the string matches... the solutions above involving the "grep" utility will get you into trouble if the old girlfriend is "Jane" and a random set of your files happen to contain the name "Janet"...

The same concern could apply to the string substitutions that you do: you should perhaps be careful to set it up like so:

s!\boldgirlfriendname\b!newgirlfriendname!g
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(and heaven help you if you ditch a girl with a name like April, May or June... ;)