anchor.diff:  
   // check if the string contains only letters (a-zA-Z).
   bool isAlpha(String str) {
  -  return alpha.hasMatch(str);
  +  return _alpha.hasMatch(str);
   }
   
   
   // check if the string contains only numbers
   bool isNumeric(String str) {
  -  return numeric.hasMatch(str);
  +  return _numeric.hasMatch(str);
   }
   
   
   // check if the string contains only letters and numbers
   bool isAlphanumeric(String str) {
  -  return alphanumeric.hasMatch(str);
  +  return _alphanumeric.hasMatch(str);
   }
   
   
   // check if a string is base64 encoded
   bool isBase64(String str) {
  -  return base64.hasMatch(str);
  +  return _base64.hasMatch(str);
   }
   
   
  
positive.code: 
  // check if the string is an email
  bool isEmail(String str) {
    return email.hasMatch(str.toLowerCase());
  }
  
  
  
negative.code: 
  // check if the string's length falls in a range
  bool isLength(String str, int min, [int max]) {
    List surrogatePairs = _surrogatePairsRegExp.allMatches(str).toList();
    int len = str.length - surrogatePairs.length;
    return len >= min && (max == null || len <= max);
  }
  