|As far as I can tell there are at least three main types; POSIX basic, POSIX extended, and Perl Compatible.
There's a list of engines here:
Comparison of regular expression engines - Wikipedia[^]
And apparently some differences between PERL and PCRE:
Perl Compatible Regular Expressions - Wikipedia[^]
I don't know/understand if let's say 80 or 97 percent of the Regex syntax is the same between one version or another or if they are distinct subtypes with significant differences.
I don't know if they all support ascii, Unicode, and utf encoding, or whether they are all capable of returning matched variables or if some are only providing a match/no match result.