SearchFilter may use invalid regex

[sevdog]

Using SearchFilter with the iregex lookup may forward an invalid regex to database backend resulting in a DataError.

With this simple setup

class MyModel(models.Model):
    field = models.CharField(max_length=255)

class MyViewSet(viewsets.ModelViewSet):
    queryset = MyModel.objects.all()
    filter_backends = (SearchFilter, )
    search_fields = ('$field', )

When querying with a "non completed" regular expression (ie (a, which has an unbalanced parens) will raise an error, since there is no validation for the proided regex before using it.

DataError at /myapi
invalid regular expression: parentheses () not balanced

When using regular expressions which may come from an untrusted source they must be validated in some way.

[sevdog]

There should be some validation in SearchFilter when lookup is iregex there are tree possible approaches:

• safe-fallback to icontans if search term is not a regex
• raise a validation error since the regex cannot match
• skip the mal-formed search term