Erlang requires doubling up on escape backslashes, which generally causes me grief even after I flail about and finally remember it. Subtleties abound.
Another strange one is the (old) Visual Studio syntax with its :i (actually often quite helpful) and $1 instead of \1 (even in the new syntax).
https://msdn.microsoft.com/en-us/library/2k3te2cs(v=vs.110)....
[1] http://www.regular-expressions.info/lookaround.html [2] http://www.regular-expressions.info/refadv.html