Small addition to add capture group (submatch) support to strings.

[chris-garrett]

[steve-todorov]

@mattfarina Hey -- is there a reason for this not to be merged yet? Sounds like a reasonable addition. :)

[ElectricWarr]

Found this while looking into adding this functionality myself, very happy to see there was already such an clear and well-written PR but absolutely crushed to discover it hasn't been merged and there's essentially no activity on this repo any more. It's a real shame!

[EinDev]

Any update to this? I desperately need this for a project and as this does not support lookbehinds, what i want to do is simply not achievable...