Added PREG_DELIM to stop warning about unknown modifier

root-based-category-urls.php

<?php
/*
 * Plugin Name: Root-based Category URLs
 * Description: Enables root-based Category URLs, i.e. Makes /category/my-category/ URLs route as /my-category/
 * Author: Mike Schinkel
 * Author URI: http://about.me/mikeschinkel
 * Plugin URI: https://gist.github.com/1421235
 * Version: 0.1.1
 * License: GPL 2+
 */
 
class Rootbased_Category_URLs {

	const PREG_DELIM = '@';

	static function on_load() {
		add_filter( 'request', array( __CLASS__, 'request' ) );
		add_filter( 'category_link', array( __CLASS__, 'category_link' ) );
	}
	
	function request( $query_vars ) {
		global $wp;
		if ( preg_match( self::PREG_DELIM . self::get_category_base() . '(.*)' . self::PREG_DELIM, $wp->request, $match ) ) {
			wp_safe_redirect( self::category_link( $wp->request ), 301 );
			exit;
		}
		$category_hash = array_flip( self::get_category_slugs() );
		if ( isset( $category_hash[$wp->request] ) ) {
			$query_vars = array( 'category_name' => $wp->request );
		}
		return $query_vars;
	}
	function get_category_base() {
		$category_base = get_option('category_base');
		if ( empty( $category_base ) )
			$category_base = 'category';
		return "/{$category_base}/";
	}
	function category_link( $category_link ) {
		return str_replace( self::get_category_base(), '/', $category_link );
	}
	function get_category_slugs() {
		$categories = get_categories();
		foreach( $categories as $index => $category ) {
			$categories[$index] = $category->slug;
		}
		return $categories;
	}
}
Rootbased_Category_URLs::on_load();