Given a set of regexps and given a wanted logical concatenation (lets start with AND,OR) of these regexps, is there any mechanism to create a smaller, more eficient set of regexps that will have the same effect?

Very unlikely given the power of Perl regular expressions. One of the questions that you would need to answer is whether two grammars (I use grammar here instead of regular expression to avoid confusion later on) are equivalent; that is, whether the will match the same language (where a language is the set of all strings matched by a grammar). For regular expressions (not Perl regular expressions, but for traditional ones) this question is decidable. But for more powerful grammars on the Chomsky hierarchy, this question is undecidable.

However, Perl regular expressions are hard to place on the Chomsky hierarchy. Without (?{ }) or (??{ }), Perl regular expressions cannot be used to create all context free grammars (take the language of strings with balanced parenthesis for instance). On the other hand, it can be used to create grammars that are context sensitive on the Chomsky hierarchy. (With (?{ }) it could very well be that Perl regular expressions are at least as powerful as context free grammars, but I don't see an obvious proof.)

My guts say that such a mechanism does not exist, that the problem is unsolvable. If the problem is solvable, it's going to be incredibly hard.

Abigail