get_shortcode_regex

function get_shortcode_regex( $tagnames = null ) {
	global $shortcode_tags;

	if ( empty( $tagnames ) ) {
		$tagnames = array_keys( $shortcode_tags );
	}
	$tagregexp = join( '|', array_map( 'preg_quote', $tagnames ) );

	// WARNING! Do not change this regex without changing do_shortcode_tag() and strip_shortcode_tag().
	// Also, see shortcode_unautop() and shortcode.js.

	// phpcs:disable Squiz.Strings.ConcatenationSpacing.PaddingFound -- don't remove regex indentation
	return '\\['                             // Opening bracket.
		. '(\\[?)'                           // 1: Optional second opening bracket for escaping shortcodes: [[tag]].
		. "($tagregexp)"                     // 2: Shortcode name.
		. '(?![\\w-])'                       // Not followed by word character or hyphen.
		. '('                                // 3: Unroll the loop: Inside the opening shortcode tag.
		.     '[^\\]\\/]*'                   // Not a closing bracket or forward slash.
		.     '(?:'
		.         '\\/(?!\\])'               // A forward slash not followed by a closing bracket.
		.         '[^\\]\\/]*'               // Not a closing bracket or forward slash.
		.     ')*?'
		. ')'
		. '(?:'
		.     '(\\/)'                        // 4: Self closing tag...
		.     '\\]'                          // ...and closing bracket.
		. '|'
		.     '\\]'                          // Closing bracket.
		.     '(?:'
		.         '('                        // 5: Unroll the loop: Optionally, anything between the opening and closing shortcode tags.
		.             '[^\\[]*+'             // Not an opening bracket.
		.             '(?:'
		.                 '\\[(?!\\/\\2\\])' // An opening bracket not followed by the closing shortcode tag.
		.                 '[^\\[]*+'         // Not an opening bracket.
		.             ')*+'
		.         ')'
		.         '\\[\\/\\2\\]'             // Closing shortcode tag.
		.     ')?'
		. ')'
		. '(\\]?)';                          // 6: Optional second closing brocket for escaping shortcodes: [[tag]].
	// phpcs:enable
}