The lesson ... if the delimiter character(s) are the same as those enclosing the regex ...

I think that the lesson should be that if a regex pattern contains any character that is the same as the regex delimiters, care should be exercised. Usually, it's enough to escape such embedded characters in the normal way, but in this particular case, any escapology does not take place within the normal context of regex interpolation, and that leads to some unexpected quirky behavior.