I like this filter overall, but I'd like to point out a minor optimization opportunity: you're using the same regex twice here, so if this were a complicated regex I would suggest using re.subn and just checking the number of times it was replaced. However, since you're just matching the start of the line, I would recommend avoiding using the regex altogether, and replacing it with the following:
```python
if url.startswith('//'):
    return url.replace('//', 'https://', 1)
```
Just a suggestion for a minor improvement in performance and readability.