A function to take the regexp-matched subsring directly

[daanturo]

How do you think about such a built-in functionality?

I find myself using them in parsing strings alot, most of the time I
just care about whether a particular matched (sub-)expression or not:
Does it match? Yes? Good, throw me result, else give me null.

The implementation is attached. I name it 'regexp-match' (please
change the name if needed).

-- 
Daanturo.

0001-Define-regexp-match-regexp-match.patch
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