kses strips tags with hyphens in the name

[batmoo]

The W3C Custom Elements spec (​http://www.w3.org/TR/custom-elements/#concepts) allows you to use your own custom DOM elements/tags. One of the main requirements is that the tag name "must contain a U+002D HYPHEN-MINUS character".

However, kses does not currently factor in the hyphen and strips out tags, even if they're set to be explicitly allowed:

$string = '<hyphenated-tag>Hello</hyphenated-tag>';
wp_kses( $string, array( 'hyphenated-tag' => array() ) );

// returns 'Hello' but should be '<hyphenated-tag>Hello</hyphenated-tag>'

This is because the regex for parsing the element name within kses only allow alphanumerics. It would be nice if this was loosened to allow hyphens in the tag name.

[batmoo]

Attached patches attempt a quick pass at this and include basic tests.

[johnregan3]

Polite bump for this one, as escaping AMP HTML tags often requires allowing hyphens, as in, <amp-ad></amp-ad>.
​https://ampbyexample.com/components/amp-ad/

[jorbin]

In 38511:

Formatting: Allow KSES custom elements with hyphens

The W3C Custom Elements spec (​http://www.w3.org/TR/custom-elements/#concepts) allows you to use your own custom DOM elements/tags. One of the main requirements is that the tag name "must contain a U+002D HYPHEN-MINUS character". This adjusts KSES to allow it.

Fixes #34105.
Props batmoo.