It’s not something that really depends on the string being matched (except that that can determine the default value to use)
You have to do tricks like I did in StrRegex, otherwise you’d be constantly recompiling.
It can be done though, it would need to compile the pattern both ways (probably lazily for the non-default cases).
I’ll make sure this is handled efficiently in StrRegex.
Basically, if you have an UTF8Str or UniStr, the default would be to have u set for the pattern, but not for ASCIIStr (similar to the PCRE2_UTF) flag, and at run-time, you pick the compiled regex to use based on the the type of the string being matched and the options given for the pattern, and if that field is C_NULL, you compile the regex, with those options, and the correct code unit size, and then call the appropriate match code.