How do I use part of a class as variable using jQuery

I've got an element like this:

<ul class="MainMenu_SubMenu m m0 mid157">

I need to get the mid157 and use that 157 for a variable for something开发者_如何学运维 else, i.e:

$('SomeOtherElement157')

So my question is how do get the class name as a variable and then strip all but the number? The classes are generated so some have more and some have less but they will all have that mid...

var className = $('ul').attr('class'),
    midNum = /\bmid(\S+)/.exec(className)[1],
    selector = 'SomeOtherElement' + midNum;

$(selector);

Works for all of the following:

<ul class="MainMenu_SubMenu m m0 mid157"></ul>
<ul class="mid158"></ul>
<ul class="mid159 MainMenu_SubMenu m m0"></ul>
<ul class="MainMenu_SubMenu mid160 m m0"></ul>

Demo: http://jsfiddle.net/mattball/P3G8H/

You can use grep:

var classes=$("div").attr("class").split(" ");

var matches = jQuery.grep(classes, function(n, i){
  return n.indexOf("mid")===0;
});

if(matches[0]){
    var number=matches[0].substring(3);

    // do something with that number 
}

var patt1   = /mid(\d+)/,
    matches = $('ul').attr('class').match(patt1);

if (matches != undefined && matches[1] != undefined) {
    $('SomeOtherElement'+matches[1]);        
} else {
    alert("No matches.");   
}

var id = $("div").attr("class").match(/mid([\d]+)/gi)[0].replace("mid", "SomethingElse");

alert(id);

http://jsfiddle.net/bkSFf/

var nums = 
    document.getElementsByTagName('ul')[0]
                .className.match(/.*mid(\d*)/)[1];

$('selector' + nums);