std.regex: fix inverted condition in Group.toString

[kubo39]

A group is matched iff begin <= end (Group.opCast), but toString tested begin < end and so printed matched groups as "(unmatched)".

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[0xEAB]

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[kubo39]

I think it's not necessary, only for debugging purpose.

[DmitryOlshansky]

Seems fine. The whole thing is mostly for debugging.