## Bug description

The following filter fails with a 500 internal error.

![2021-02-04_12-07](https://user-images.githubusercontent.com/148820/106877644-a9f4ce80-66e1-11eb-8320-87bb213bdde2.png)


## Expected behavior

Error is shown to user about invalid regular expression.

Note no errors shown on the page as well.

## How to reproduce

1. Go to events
2. Add filter "Current URL matches regular expression ?*"
3. See infinite loading screen / error

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: _please provide_

## Additional context

Note this can also happen when filtering by events.

#### *Thank you* for your bug report – we love squashing them!

ee/clickhouse/models/property.py
for idx, prop in enumerate(filters):
        if prop.type == "cohort":
            cohort = Cohort.objects.get(pk=prop.value)
            person_id_query, cohort_filter_params = format_filter_query(cohort)
            params = {**params, **cohort_filter_params}
            final.append(
                "AND {table_name}distinct_id IN ({clause})".format(table_name=table_name, clause=person_id_query)
            )
        elif prop.type == "person":
            filter_query, filter_params = prop_filter_json_extract(
                prop, idx, "{}person".format(prepend), allow_denormalized_props=allow_denormalized_props
            )
            final.append(
                "AND {table_name}distinct_id IN ({filter_query})".format(
                    filter_query=GET_DISTINCT_IDS_BY_PROPERTY_SQL.format(filters=filter_query), table_name=table_name
                )
            )
            params.update(filter_params)
        else:
            filter_query, filter_params = prop_filter_json_extract(
                prop,
                idx,
                prepend,
                prop_var="{}properties".format(table_name),
                allow_denormalized_props=allow_denormalized_props,
            )
            final.append(f"{filter_query} AND {table_name}team_id = %(team_id)s" if team_id else filter_query)
            params.update(filter_params)
    return " ".join(final), params
def prop_filter_json_extract(