Source

File: wp-includes/kses.php

function wp_kses_hair_parse( $attr ) {
	if ( '' === $attr ) {
		return array();
	}

	// phpcs:disable Squiz.Strings.ConcatenationSpacing.PaddingFound -- don't remove regex indentation
	$regex =
		'(?:'
		.     '[_a-zA-Z][-_a-zA-Z0-9:.]*' // Attribute name.
		. '|'
		.     '\[\[?[^\[\]]+\]\]?'        // Shortcode in the name position implies unfiltered_html.
		. ')'
		. '(?:'               // Attribute value.
		.     '\s*=\s*'       // All values begin with '='.
		.     '(?:'
		.         '"[^"]*"'   // Double-quoted.
		.     '|'
		.         "'[^']*'"   // Single-quoted.
		.     '|'
		.         '[^\s"\']+' // Non-quoted.
		.         '(?:\s|$)'  // Must have a space.
		.     ')'
		. '|'
		.     '(?:\s|$)'      // If attribute has no value, space is required.
		. ')'
		. '\s*';              // Trailing space is optional except as mentioned above.
	// phpcs:enable

	// Although it is possible to reduce this procedure to a single regexp,
	// we must run that regexp twice to get exactly the expected result.

	$validation = "%^($regex)+$%";
	$extraction = "%$regex%";

	if ( 1 === preg_match( $validation, $attr ) ) {
		preg_match_all( $extraction, $attr, $attrarr );
		return $attrarr[0];
	} else {
		return false;
	}
}