Regular expression to detect semi-colon terminated C++ for & while loops

This is a great example of using the wrong tool for the job. Regular expressions do not handle arbitrarily nested sub-matches very well. What you should do instead is use a real lexer and parser (a grammar for C++ should be easy to find) and look for unexpectedly empty loop bodies.

You could write a little, very simple routine that does it, without using a regular expression:

• Set a position counter pos so that is points to just before the opening bracket after your for or while.
• Set an open brackets counter openBr to 0.
• Now keep incrementing pos, reading the characters at the respective positions, and increment openBr when you see an opening bracket, and decrement it when you see a closing bracket. That will increment it once at the beginning, for the first opening bracket in "for (", increment and decrement some more for some brackets in between, and set it back to 0 when your for bracket closes.
• So, stop when openBr is 0 again.

The stopping positon is your closing bracket of for(...). Now you can check if there is a semicolon following or not.

This is the kind of thing you really shouldn't do with a regular expression. Just parse the string one character at a time, keeping track of opening/closing parentheses.

If this is all you're looking for, you definitely don't need a full-blown C++ grammar lexer/parser. If you want practice, you can write a little recursive-decent parser, but even that's a bit much for just matching parentheses.