Update Rust crate regex to 1.8.4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
regex	dependencies	patch	1.8.3 -> 1.8.4

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Release Notes

rust-lang/regex

v1.8.4

Compare Source

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This is a patch release that fixes a bug where (?-u:\B) was allowed in
Unicode regexes, despite the fact that the current matching engines can report
match offsets between the code units of a single UTF-8 encoded codepoint. That
in turn means that match offsets that split a codepoint could be reported,
which in turn results in panicking when one uses them to slice a &str.

This bug occurred in the transition to regex 1.8 because the underlying
syntactical error that prevented this regex from compiling was intentionally
removed. That's because (?-u:\B) will be permitted in Unicode regexes in
regex 1.9, but the matching engines will guarantee to never report match
offsets that split a codepoint. When the underlying syntactical error was
removed, no code was added to ensure that (?-u:\B) didn't compile in the
regex 1.8 transition release. This release, regex 1.8.4, adds that code
such that Regex::new(r"(?-u:\B)") returns to the regex <1.8 behavior of
not compiling. (A bytes::Regex can still of course compile it.)

Bug fixes:

• BUG #​1006:
  Fix a bug where (?-u:\B) was allowed in Unicode regexes, and in turn could
  lead to match offsets that split a codepoint in &str.

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