oEmbed discovery fails on XHTML head links

[cweiske]

In XHTML, link tags in the HTML head need to be closed:

<link title="test" href="​http://example.org/test"/>

Wordpress' oEmbed discovery in WP_oEmbed::discover() fails on this.
It extracts the attributes ( title="test" href="http://example.org/test"/) and sends it through shortcode_parse_atts. The trailing slash is included even though it's not part of the attributes.

Either discover()'s link tag should not match the slash, or shortcode_parse_atts should be changed to cope with it.

[cweiske]

Patch that fixes the problem.

[wonderboymusic]

In 31407:

oEmbed discovery fails on XHTML head links, adjust the regex to not match /.

Props cweiske.
Fixes #31212.

[nacin]

In general, I'd argue that /U (PCRE_UNGREEDY) should not be used. It isn't natural to read a regular expression as if things are un-greedy; using ? to make something lazy/reluctant/nongreedy is going to be easier to understand.

/U is also sometimes misread as /u which is for UTF-8 (and also should not be used in many cases).