The SocialSecurityNumberValidator class validates that a String is a valid United States Social Security number. It does not check whether it is an existing Social Security number.

class SocialSecurityNumberValidator extends WidgetValidator {
 
 /**
  * Specifies the set of formatting characters allowed in the input.
  * By default equals "-"
  */
 String allowedFormatChars;
 
 /**
  * Error message when the value contains characters other than digits and 
  * formatting characters defined by the [allowedFormatChars] property.
  * By default equals "You entered invalid characters in your Social Security number."
  */
 String invalidCharError;
 
 /**
  * Error message when the value is incorrectly formatted.
  * By default equals "The Social Security number must be 9 digits or in the form NNN-NN-NNNN."
  */
 String wrongFormatError;
 
 /**
  * Error message when the value contains an invalid Social Security number.
  * By default equals "Invalid Social Security number; not allowed all zeros in any digit group."
  */
 String zeroGroupError;
 
 /**
  * Error message when the value contains an invalid Social Security number.
  * By default equals "Invalid Social Security number; not allowed Area Numbers 666 and 900-999."
  */
 String wrongAreaError;
 
 SocialSecurityNumberValidator(Widget widget, {
   this.allowedFormatChars:"-",
   this.invalidCharError:"You entered invalid characters in your Social Security number.",
   this.wrongFormatError:"The Social Security number must be 9 digits or in the form NNN-NN-NNNN.",
   this.zeroGroupError:"Invalid Social Security number; not allowed all zeros in any digit group.",
   this.wrongAreaError:"Invalid Social Security number; not allowed Area Numbers 666 and 900-999."
   }) : super(widget);
 
 /**
  *  Executes the validation logic of this validator to validate a Social Security number. 
  *  For an invalid result returns an List of [ValidationResult] 
  *  objects, with one ValidationResult object for each field examined by the 
  *  validator that failed validation.
  *
  *  You must override this method in a subclass of a validator class.
  */
 List<ValidationResult> doValidation(dynamic value) {
   List<ValidationResult> results = super.doValidation(value);

   // Return if there are errors or if the required property is set to false 
   // and length is 0.
   String val = value != null ? value.toString() : "";
   
   if (results.length > 0 || ((val.length == 0) && !required))
     return results;
   else
     return _validateSocialSecurity(value);
 }
 
 /**
  * Validate [value] and  return List of [ValidationResult] objects, with one
  * ValidationResult object for each field examined by the validator.
  */
 List<ValidationResult> _validateSocialSecurity(dynamic value) {
   List<ValidationResult> results = new List<ValidationResult>();
   
   // Resource-backed properties of the validator.
   int len = value.length;
   bool checkForFormatChars = false;

   if ((len != 9) && (len != 11)) {
     results.add(new ValidationResult(true, wrongFormatError));
     return results;
   }

   for (int i = 0; i < allowedFormatChars.length; i++) {
     if (Validator.DECIMAL_DIGITS.indexOf(allowedFormatChars[i]) != -1) {
       throw new Exception("Invalid format chars");
     }
   }

   if (len == 11)
     checkForFormatChars = true;

   for (int i = 0; i < len; i++) {
     String allowedChars;
     if (checkForFormatChars && (i == 3 || i == 6))
       allowedChars = allowedFormatChars;
     else
       allowedChars = Validator.DECIMAL_DIGITS;

     if (allowedChars.indexOf(value[i]) == -1) {
       results.add(new ValidationResult(true, invalidCharError));
       return results;
     }
   }
   
   String _value = value.replaceAll("-", "");
   String area = _value.substring(0, 3);
   String group = _value.substring(3, 5);
   String serial = _value.substring(5, 9);

   // Not allowed are SSNs with all zeros in any digit group 
   // (000-xx-####, ###-00-####, ###-xx-0000).
   if (area == "000" ||
       group == "00" ||
       serial == "0000") {
     results.add(new ValidationResult(true, zeroGroupError));
     return results;
   }
   
   // Not allowed are SSNs with Area Numbers 000, 666 and 900-999.
   int areaNumber = int.parse(area);
   if (areaNumber == 666 ||
       (areaNumber >= 900 && areaNumber <= 999)) {
     results.add(new ValidationResult(true, wrongAreaError));
     return results;
   }
   
   return results;
 }
}