[m463]

In my example, if you over-escape the first period or under-escape the second period the regex will undermatch or overmatch.

for me it took a few tries to get filexc to not match and file.c to match in my example in the 3 languages.

[logicallee]

My comment was just about what you call "under-escap[ing] the second period."

I mean if you don't know if a ,;:"%$$@!_€|~ or any other character means something you can just write \ before that character. In other words, without thinking of whether it means something in your regex language. I don't think = means anything, but I would write \= to match an equals sign, so I don't even have to think about it. So my comment was about matching literal characters of any kind. I would have used a \ for the literal period out of habit, just because it wasn't [A-Za-z]. So my version would have been right the first time.