Validating a string...

Hello-

Here is my problem.  I'm trying to parse a string that is entered by a user to make it conform to certain rules.

For example I needed to do this with a phone number so the function would strip out all of the non-numeric characters.  White space and "-", "(", and so-on.

My function that accomplishes this is

void TForm1::StripPhoneNumber(char * dest, char * src)
{
  while(*src)
  {
    if(isdigit(*src))
    {
      *(dest++) = *src;
    }

    src++;
  }

  *dest = 0;
}

It works well, but now i'm trying to do something similar but that keeps all letters and digits.  For instance,
1-800-ABC-DEFG would still return 1800ABCDEFG, and not just 1800.  

All I really need is an isletter equivalent to isdigit to put in the if statement.  Does such a function exist?

Thanks!