Port nim regex tests

[stephentoub]

Nim has lots of regex functional tests we should port over to improve our coverage:
https://github.com/nitely/nim-regex/blob/eeefb4f51264ff3bc3b36caf55672a74f52f5ef5/tests/tests.nim

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Issue Details

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Nim has lots of regex functional tests we should port over to improve our coverage:
https://github.com/nitely/nim-regex/blob/eeefb4f51264ff3bc3b36caf55672a74f52f5ef5/tests/tests.nim

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	.NET 7.0

[joperezr]

This will likely miss ZBB but will still make it before 7.0. Adjusting the milestone just because this is not technically a blocker for 7.0.