I have a string of characters "PME". I only want the string to be valid if it matches PME exactly (in that order) and must be exactly three letters long (case insensitive). In short, only the string PME will work, no variations, and no shorter.
I have this so far:
This will ensure that PME is in the correct sequence, but something like PM works also which I can't have. I tried:
- [p(?=m(?=e))P(?=M(?=E))]{1}
However that doesn't seem to work.