If current page url

 $(document).ready(function() {

$(function(){

    var url = window.location.pathname, 
        urlRegExp = new RegExp(url.replace(/\/$/,'') + "$"); // create regexp to match current url pathname and remove trailing slash if present as it could collide with the link in navigation in case trailing slash wasn't present there
        // now grab every link from the navigation

		$('.sub_nav a').each(function(){
            // and test its normalized href against the url pathname regexp
            if(urlRegExp.test(this.href.replace(/\/$/,''))){
                $(this).addClass('sub_nav_active');
            }
        });

});
	
	   });

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