Alex has a valid point in relation to PEP 399, since, like lzma, regex will be coming in under the "special permission" clause that allows the addition of C extension modules without pure Python equivalents. Unlike lzma, though, the new regex engine isn't a relatively simple wrapper around an existing library - supporting the new API features on other implementations is going to mean a substantial amount of work.

In practice, I expect that a pure Python implementation of a regular expression engine would only be fast enough to be usable on PyPy. So while we'd almost certainly accept a patch that added a parallel Python implementation, I doubt it would actually help Jython or IronPython all that much - they're probably going to need versions written in Java and C# to be effective (as I believe they already have for the re module).