If current page url

For one your code structure is a bit odd

$(document).ready(function() {
$(function(){
...

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These are effectively the same - use one or the other - not both

$(function() {
...
});

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This does not make sense - why are you using RegEx on a static string - when an '==' will do the same thing?

urlRegExp = new RegExp(url.replace(/\/$/,'') + "$")

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Here is a stab at doing what you want

$(function() {
  // Get the pathname
  var url = window.location.pathname;

  // If the last char is a '/' then strip it 
  url = (url.substr(-1) === '/') ? url.slice(0, url.length-1) : url;

  // Match the <a> with an href attr that matches the url
  $('a[href="' + url +'"]').addClass('sub_nav_active');
});

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The sample code below demonstrates the concept - working version here
HTML

      <div class="sub_nav">
        <ul>
           <li><a href="/aboutus/values">Values</a></li>
           <li><a href="/aboutus/diversity">Diversity</a></li>                 
        </ul>
      </div> 

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JavaScript

$(function() {
  $('.tester').click(function(e) {
    e.preventDefault();
    url = $('#url').val();
	url = (url.substr(-1) === '/') ? url.slice(0, url.length-1) : url;
    $('.sub_nav_active').removeClass('sub_nav_active');
    $('.sub_nav a[href="' + url +'"]').addClass('sub_nav_active');
  });
});

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