for Regexp validator set html input tag 'pattern' attribute (used if …

…novalidate not set on a form)

src/wtforms/validators.py

@@ -341,6 +341,7 @@ def __init__(self, regex, flags=0, message=None):
             regex = re.compile(regex, flags)
         self.regex = regex
         self.message = message
+        self.field_flags = {"pattern": regex.pattern}
 
     def __call__(self, form, field, message=None):
         match = self.regex.match(field.data or "")