Add Class to first Paragraph in WordPress the_content();

functions.php

/* Add Class to first Paragraph in WordPress the_content();
   Source: http://webdevbits.com/wordpress/add-class-to-first-paragraph-in-wordpress-the_content/
   ------------------------------------------------------------------------------------------- */

function first_paragraph($content){
  if ( is_page() || ('school' == get_post_type() ) ) {
    return preg_replace('/<p([^>]+)?>/', '<p$1>', $content, 1);
  } else {
    return preg_replace('/<p([^>]+)?>/', '<p$1 class="intro">', $content, 1);
  }
}
add_filter('the_content', 'first_paragraph');