[eru]

It doesn't have to be slower at runtime.

Parsec will by default have to be slower, because it allows eg infinite backtracking / lookahead. If you use a parser combinator library that has less power, you can make it faster. E.g. if your library only has to exprose on Applicative interface.

I am working on a Parsec-like library for parsing regular languages (as in theoretical computer sience, not as in Perl). As opposed to grep I want to do something with the results instead of just accepting / rejecting, and I also want to expose more operators under which regular languages are closed, like difference or intersection (i.e. parallel match) or matching all elements of a set once but in any order, or chopping off a regular prefix or suffix.