How can I improve comma insertion code?

Here is the code I have written. It works fine except if the input is a negative six-digit number, a comma appears just after the negative sign like so: -,963,562

This is part of a larger program, obviously, and one of the other things I need to do is have negative numbers like -45 recognized as valid but numbers like 4-5 not recognized as valid. I hope this is enough information for someone to help me.

Code:

string commaString()
       {
            if (validInput = true);
            {
            string comString = inString;
            int length = comString.length();


            int i = (comString.length() - 3);
            while (i > 0)
            {
                comString.insert(i, 1, ',');
                i = i - 3;
            }


            return comString;
            }
        
       }

>>if (validInput = true);
You are assigning instead of comparing.
You also have an erroneous ending semicolon which should not be there.

Are you writing some parsing program? Then it might be good to understand how parsing is typically done. You write a regular expression for what is syntactically correct. Then you translate that into a NFA, and then from that, a DFA. These automatons will essentially accept or reject what syntax is valid or not.
From that, you would need a parser to parse the actual meaning of your syntax, which is typically an LL parser or LR parser. That also means you have to define some grammar for your language.
There are tools out there that does all this for you. LLVM might be a good place to look, I think, if you are interested.