Hi Erik Malcolm Wallace describes a commit combinator in the paper "Partial parsing: combining choice with commitment" which sounds like what you would want. It is implemented for Polyparse rather than Parsec though.
From a quick scan of the paper and code, the implementation appears to be built into the Parser type, so it is a primitive rather than a definable combinator.
If your Parsec parser uses try because it is not especially left-factored, one extra combinator I have found useful for left-factoring on the cheap is optionalSuffix: optionalSuffix :: (a -> c) -> (a -> b -> c) -> Parser a -> Parser b -> Parser c optionalSuffix f g pbody psuffix = do a <- pbody mb_b <- optionMaybe psuffix return $ maybe (f a) (\b -> g a b) mb_b