wp_kses wp_kses_hair fails to allow a valueless attribute when is follwed by /

[luistar15]

I have created a static gutenberg block for managing forms.
The block generates <input> html elements, for example:

<input name="email" required/>

But after saving the post, the required attribute is striped in the db:

<input name="email"/>

So the visitors are sendig the form ignoring the validation.

It only happens when the attribute is the last one and is followerd by / without spaces:

<input name="email" required>    => OK
<input name="email" required/>   => ERROR
<input name="email" required />  => OK

I put togheter a test case:

// theme/functions.php

add_action( 'init', function () {
        $tests = [
                '<input name="email" required>',
                '<input name="email" required/>',
                '<input name="email" required />',
        ];

        $allowed_html = [
                'input' => [
                        'name'     => true,
                        'required' => true,
                ],
        ];

        header('Content-Type: text/plain; charset=UTF-8', true);

        foreach ( $tests as $test ) {
                $sanitized = wp_kses( $test, $allowed_html );
                printf( "_in => %s\nout => %s\n\n", $test, $sanitized);
        }

        exit;
});

I have found where the error could be fixed:

// wp-includes/kses.php:1320

if ( preg_match( '/^\s+/', $attr ) ) { // Valueless.

Replacing the regex /^\s+/ with /^(\s+|\/$)/ fixes the problem.

I'm hopping it can be fixed for the next release, until then I'm manually applying the patch again.

wp_kses_hair fails to parse a valueless attribute at the end of the list.
For example: <input name="email" required/> will strip the required attribute.
This change fixes it.

https://core.trac.wordpress.org/ticket/56521