SQL group by middle part of string

I have string column that looks usually approximately like this:

https://mapy.cz/zakladni?x=16.3360208&y=49.6718038&z=8&source=firm&id=13123554

I would like to group data by source which is part of the string – four letters behind “source=” (in the case above: firm) and then simply count them. Is there a way to achieve this directly in SQL code?