Backreferences and look around.
I don't think it's proven that they can't be added on to an automata based regular expression engine. Nobody has figured it out yet though.
I don't think it's proven that they can't be added on to an automata based regular expression engine. Nobody has figured it out yet though.
The various features in PERL allow you to emulate context-free languages. There is a proof somewhere, but it's trivial to see you can use backreferences to parse languages with well-nested parens.
Languages of well-nested parens are known to be non-regular, and thus impossible to write with regular expressions.
Also note that knowing if an arbitrary context-free language is regular is undecidable. That doesn't mean you can't try to optimize down some PERL grammar to a regex, but your optimization will never be complete.
Are you reffering to the programming language or PCRE? I would expect PCRE to be a simple superset of the regex grammar, and thus the optimization is complete and trivial: the appearance of any PCRE constructs denies the use of the DFA engine; otherwise, use the DFA.
I don’t see how you could ever optimize out eg a backreference (converting to some equivalent regex) without already having parsed the subject text and determining it to be unnecessary. In which case, I don’t see how the inability to determine regularity of a context-free grammar is relevant (if you’re referring to the context-free nature of PCRE’s grammar, its not clear to me why you should care where the regex input grammar itself is regular/context-free, and why you’d want to convert PCRE to a regular grammar; optimizing a regex search shouldn’t care about the grammar defining the search)