function Validate_String(string, return_invalid_chars) {
  valid_chars = '1234567890-_.^~abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
  invalid_chars = '';
  if(string == null || string == '')
     return(true);

//For every character on the string.   
  for(index = 0; index < string.length; index++) {
    char = string.substr(index, 1);                        
     
    //Is it a valid character?
    if(valid_chars.indexOf(char) == -1) {
      //If not, is it already on the list of invalid characters?
      if(invalid_chars.indexOf(char) == -1) {
        //If it's not, add it.
        if(invalid_chars == '')
          invalid_chars += char;
        else
          invalid_chars += ', ' + char;
      }
    }
  }
            
  //If the string does not contain invalid characters, the function will return true.
  //If it does, it will either return false or a list of the invalid characters used
  //in the string, depending on the value of the second parameter.
  if(return_invalid_chars == true && invalid_chars != '') {
    last_comma = invalid_chars.lastIndexOf(',');
    if(last_comma != -1)
      invalid_chars = invalid_chars.substr(0, $last_comma) + 
      ' and ' + invalid_chars.substr(last_comma + 1, invalid_chars.length);
    return(invalid_chars);
    }
  else
    return(invalid_chars == ''); 
}


function Validate_Email_Address(email_address) {
  //Assumes that valid email addresses consist of user_name@domain.tld
  at = email_address.indexOf('@');
  dot = email_address.indexOf('.');

  if(at == -1 || 
    dot == -1 || 
    dot <= at + 1 ||
    dot == 0 || 
    dot == email_address.length - 1)
    return(false);
     
  user_name = email_address.substr(0, at);
  domain_name = email_address.substr(at + 1, email_address.length);                  

  if(Validate_String(user_name) === false || 
    Validate_String(domain_name) === false)
    return(false);                     

  return(true);
}