Since davido and johngg solved the problem, I won't bother with that. Especially as davido's suggestion was much shorter than I was going to write... 8^)
But as I've done basically the same task for a client, I thought I'd add one thing: You may want to consider putting guard strings before and after your string, and modifying the regex to require the guard strings. Appropriate guard string selection can help you avoid changing more things than you intended to in your program.
This may not be a problem in your case, but in our case, we didn't want to inadvertently change data strings along with our serial number.