Keep original pattern as attribute

yangson/constraint.py

@@ -126,6 +126,7 @@ class Pattern(Constraint):
         """Initialize the class instance."""
         super().__init__(error_tag, error_message if error_message else
                              "pattern '{}'".format(pattern))
+        self.pattern = pattern
         self.invert_match = invert_match
         try:
             self.regex = re.compile(XMLToPython(pattern))