I've always been a stickler for being specific about which regex language your thing accepts, and whether it is to match any substring, or a prefix, or a suffix, or the whole thing, or a line, or a substring of a line, or whatever.

Here are some of the [more popular][1] ones, and then there are PCRE and Python.

It took me a while to learn that some of the older ones you see in e.g. grep are [specified by POSIX][2].

[1]: https://cppreference.com/cpp/regex#Regular_expression_gramma...

[2]: https://pubs.opengroup.org/onlinepubs/009696899/basedefs/xbd...