Automatic regex builder

Question

I have N strings. Also, there are K regular expressions, unknown to me. Each string is either matching one of the regular expressions, or it is garbage. There are total of

Answer 1

The key words in academia are "grammatical inference". Unfortunately, there aren't any efficient, general algorithms to do the sort of thing you're proposing. What's your real problem?

Edit: it sounds like you might be interested in Data Description Languages. PADS (http://www.padsproj.org/) is a typical example.