Catch situation where Regex flag fails (#1426)

* Catch an exception where a user supplied regex Flag can fail to parse
* Starts on #1425

CTFd/plugins/flags/__init__.py

@@ -49,10 +49,14 @@ class CTFdRegexFlag(BaseFlag):
         saved = chal_key_obj.content
         data = chal_key_obj.data
 
+        try:
             if data == "case_insensitive":
                 res = re.match(saved, provided, re.IGNORECASE)
             else:
                 res = re.match(saved, provided)
+        # TODO: this needs plugin improvements. See #1425.
+        except re.error:
+            return False
 
         return res and res.group() == provided