rust-regex: fix issues with regex anchoring and performance
It turns out that the way I tried to work around `regex`'s behavior difference
with `re2` and Python's `re` was 1) buggy and 2) much more complicated than
needed.
In a few words:
`regex` adds `.*` on either side of patterns when no start or end anchor is
present. My previous workaround put `^` or `$` for every pattern, which is
wrong even without the other 2 bugs on top of it.
Using `^(?:<patterns>)` right at the end of the `regex` path fixes the issue.
I've opened an issue to get a build option instead:
https://github.com/rust-lang/regex/issues/675
Differential Revision: https://phab.mercurial-scm.org/D8506
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