There's a logic error in PCRE versions 8.42+ that can cause some valid PCRE regex start verbs to cause a regex compile error. It just so happens that we don't run into this bug on PCRE 8.42 with the regexes in our unit tests, but it was revealed while upgrading to version 8.45 in SERVER-58705 because the order of the items start_options array had changed.

Additionally, multiple start verbs is legal in PCRE, but that also would not have worked properly in all cases because any verbs after the first would have been wrapped in a non-capturing group, which is what causes the above error as well.