Running the tool under perf<regex::backtrack::Bounded<'a, 'm, 'r, 's, I>>::backtrack is often near the top of the profile. As result I tweaked one of the regexes to be a little more specific and hopefully reduce backtracking.
test user_agent::tests::bench_parse ... bench: 46,954 ns/iter (+/- 883)
test user_agent::tests::bench_parse ... bench: 32,635 ns/iter (+/- 936)
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