I get a warning which says "Unknown escape sequence '\.' . Its the same case with any other special character or even '\s' .

I guess I'm missing something very simple.

Yep: escapes are parsed first by the compiler *before* the regex engine gets to see the string, the compiler is complaining because \. is not a valid escape sequence for strings in C++. You want to use "\\." so that a literal '\' gets passed through to the regex engine to see: it then handles the '\.' sequence as a regex. John.