wat.php

  $pattern =
		  '/'
		. '<p>'                              // Opening paragraph
		. '\\s*+'                            // Optional leading whitespace
		. '('                                // 1: The shortcode
		.     '\\['                          // Opening bracket
		.     "($tagregexp)"                 // 2: Shortcode name
		.     '\\b'                          // Word boundary
		                                     // 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
		.     ')*?'
		.     '(?:'
		.         '\\/\\]'                   // Self closing tag and closing bracket
		.     '|'
		.         '\\]'                      // Closing bracket
		.         '(?:'                      // 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
		.         ')?'
		.     ')'
		. ')'
		. '\\s*+'                            // optional trailing whitespace
		. '<\\/p>'                           // closing paragraph
		. '/s';

	return preg_replace( $pattern, '$1', $pee );