Simple one file router for WordPress (can be used as a mu-plugin)

routes.php

<?php
/*
 * Plugin Name: Routes for WordPress
 * Description: Routes class provider plugin
 * Version:     2.0
 * Author:      Zachary Scott
 */

namespace zacscott;

/**
 * Utility to build custom routes within WordPress. Routes are defined as 
 * regular expressions and pass defined arguments to the callback function.
 *
 * Example of setting up a route:
 *
 *  Routes::add( array(
 *      'regex'    => '^myroute/([0-9]+)/?$',
 *      'callback' => array( $this, 'my_route_callback' )
 *  ) );
 *
 * The callback would then look like the following:
 *
 * function my_route_callback( $id ) {
 *   // Do things here
 * }
 * 
 * @version 2.0
 * @author Zachary Scott <zac@zacscott.net>
 */
class Routes {

	// the registered routes
	private static $routes = array( );

	// whether the setup() method has been called yet
	private static $is_setup = false;

	/**
	 * Registers a new route.
	 *
	 * @param $regex string The regular expression to match the route path.  Can contain groups which are passed as
	 * params to the callback.
	 * @param $callback function Callback for when the route is triggered
	 */
	public static function add( $args ) {
		assert( ! empty( $args ) );
		
		self::setup();
		
		// Set defaults so stuff does not break
		$args = array_merge( array(
			'regex'    => '',
			'callback' => array(),
			'title'	   => '',
		), $args );
		
		// Add to the list of routes
		if ( ! empty( $args['regex'] ) && ! empty( $args[ 'callback' ] ) ) {
			self::$routes[] = $args;
		}

	}

	// Sets up the routing system
	static function setup() {
		
		// Only setup once
		if ( self::$is_setup ) {
			return;
		}

		// Register the setup class
		add_filter( 'do_parse_request', array( __CLASS__, 'handle_routes' ), 1, 3 );
		
	}

	// Handles routing on 'do_parse_request'
	static function handle_routes( $continue, $wp, $extra_query_vars ) {

		// Get the request path / URI
		$request_path = isset( $_SERVER['REQUEST_URI'] ) ? $_SERVER['REQUEST_URI'] : '';
		$request_path = trim( $request_path, '/' );
		$request_path = strtok( $request_path, '?' );

		// Regsiter each of the routes
		foreach ( self::$routes as $route ) {
			$regex = $route['regex'];
			$callback = $route['callback'];
	
			// Regex the request path
			$matches = array();
			$match = preg_match( '{' . $regex .'}', $request_path, $matches );
	
			// Dispatch route if a hit
			if ( ! empty( $matches ) ) {
				
				// Set title if one was provided
				$route_title = $route['title'];
				add_filter( 'wp_title', function( $title, $sep, $seplocation ) use ( $route_title ) { 
					
					$title = $route_title;
					$site_title = get_bloginfo( 'name' );
					
					// Grab the separate from Yoast if defined
					if ( function_exists( 'wpseo_replace_vars' ) ) {
						$sep = wpseo_replace_vars( '%%sep%%', array() );
					}
					
					// Build the title string, basedo nthe separator
					if ( $seplocation == 'right' ) {
						$title = "{$title} {$sep} {$site_title}";
					} else {
						$title = "{$site_title} {$sep} {$title}";
					}
					
					return $title;
					
				}, 10, 3 );

				array_shift( $matches );
				call_user_func_array( $callback, $matches );
				exit;
				
			}
			
		}
		
		// No matching route found, fallback to WordPress
		return $continue;

	}

}