As Weston found, something like:
<a href=" https://wp.org"> would have
a validation error.

Those trim( $url ) calls were passed
to parse_protocol().
So do something similar inside parse_protocol(),
don't match leading space.

There is a matching group of ([^/]),
so there probably isn't a need for the lookahead.

Instead of changing this,
simply trim() the URL before passing it
to parse_protocol()

As Weston mentioned, there might not be a need
for it to run earlier.

parse_protocol() no longer trims
space on its own.

Use this instead of trim()
for the URLs in attributes.

Use this from the AMP validator,
instead of a simple regex, also use trim().

The behavior of the new parse_url_ method
is changed, so these can change back.