Add a UUID string validator

The `uuid` validator uses a regex validator to match on all UUID
strings allowed by the `uuid.UUID` class.

colander/__init__.py

@@ -300,9 +300,9 @@ class Regex(object):
         validation succeeds; otherwise, :exc:`colander.Invalid` is
         raised with the ``msg`` error message.
     """
-    def __init__(self, regex, msg=None):
+    def __init__(self, regex, msg=None, flags=0):
         if isinstance(regex, string_types):
-            self.match_object = re.compile(regex)
+            self.match_object = re.compile(regex, flags)
         else:
             self.match_object = regex
         if msg is None:
@@ -465,6 +465,11 @@ URL_REGEX = r"""(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9
 
 url = Regex(URL_REGEX, _('Must be a URL'))
 
+
+UUID_REGEX = r"""^(?:urn:uuid:)?\{?[a-f0-9]{8}(?:-?[a-f0-9]{4}){3}-?[a-f0-9]{12}\}?$"""
+uuid = Regex(UUID_REGEX, _('Invalid UUID string'), 2) # 2 = re.IGNORECASE
+
+
 class SchemaType(object):
     """ Base class for all schema types """
     def flatten(self, node, appstruct, prefix='', listitem=False):