> I guess you want the regex engine Ignoring compatibility concerns, I would want the regex engine to behave the same way RE2, Go's regexp package and Rust's r

> I guess you want the regex engine Ignoring compatibility concerns, I would want the regex engine to behave the same way RE2, Go's regexp package and Rust's regex engine behave. I remember specifically considering Cox's decision ~10 years ago when writing the initial implementation of the regex crate. I thought Perl's (and Python's) behavior on this point was whacky then and I still think it's whacky now. So I followed RE2's semantics. The OP is right to be surprised by this. And folks will continue to be surprised by it for eternity because it's an extremely subtle corner case that doesn't have a consistent story explaining its behavior. (I know you have proffered one, but I don't find it consistent in the context of a general purpose regex engine that searches arbitrary strings and not just lines.) Of course, compatibility is a trump card here. I've acknowledged that. Changing this behavior now would be too hard. The best you could probably do is some kind of migration, where you provide the more "sensible" behavior behind an opt-in flag. And then maybe Python 4 enables it by default. But it's a lot of churn, and while people will continue to be confounded by this so long as the behavior exists, it probably isn't a Huge & Common Deal In Practice. So it may not be worth fixing. But if you're starting from scratch? Yes, please don't implement $ this way. It should match the end of the string when 'm' is disabled and the end of any line (including end of string and possibly being Unicode aware, depending on how much you care about that) when 'm' is enabled.