It is very hard to see how a hard coded regex would suddenly become invalid on a second pass. Indeed, it can't. At least not without something very, very unusual happening.

I think you need to post the complete script that demostrates the problem, because your description of the problem does not make great deal of sense, and without being able to run the same code you are running and thereby reproduce the same errors you are encountering, there is no real way to help you.