wpquestions.com, ID = 2838

<?php
/*
 * Template name: Update posts regex
 */

// query for posts
query_posts( array(
    'post_type' = 'any', // get all post types
    'posts_per_page' = -1, // all posts
    'nopaging' = true // no pagination
    )
);

// loop
if ( have_posts() ) : while ( have_posts() ) : the_post();

    // set vars
    $first_match_pattern = '/\[tab:.+?\]\r/g'; // regex pattern to match
    $pattern = '/\[tab:.+?\]/g'; // regex pattern to match
    $replacement = '<!-- nextpage -->'; // replacement string
    $string = get_the_content(); // post content

    // remove the first match (along with the linebreak)
    $updated_content = preg_replace( $first_match_pattern,'' , $string, 1 );
    $string = $updated_content;
    // match and replace the rest
    $updated_content = preg_replace( $pattern, $replacement, $string );

    // Set new post content
    $my_post = array();
    $my_post['ID'] = get_the_ID();
    $my_post['post_content'] = $updated_content;

    // Update the post into the database
    $result = wp_update_post( $my_post );

endwhile; endif;

?>