对于这个特定的例子，

 var thenum = thestring.replace( /^D+/g, ''); // replace all leading non-digits with nothing

一般情况下:

 thenum = "foo3bar5".match(/d+/)[0] // "3"

由于这个答案出于某种原因广受欢迎，这里有一个奖励:正则表达式生成器.

Since this answer gained popularity for some reason, here's a bonus: regex generator.

function getre(str, num) {
  if(str === num) return 'nice try';
  var res = [/^D+/g,/D+$/g,/^D+|D+$/g,/D+/g,/D.*/g, /.*D/g,/^D+|D.*$/g,/.*D(?=d)|D+$/g];
  for(var i = 0; i < res.length; i++)
    if(str.replace(res[i], '') === num) 
      return 'num = str.replace(/' + res[i].source + '/g, "")';
  return 'no idea';
};
function update() {
  $ = function(x) { return document.getElementById(x) };
  var re = getre($('str').value, $('num').value);
  $('re').innerHTML = 'Numex speaks: <code>' + re + '</code>';
}

<p>Hi, I'm Numex, the Number Extractor Oracle.
<p>What is your string? <input id="str" value="42abc"></p>
<p>What number do you want to extract? <input id="num" value="42"></p>
<p><button onclick="update()">Insert Coin</button></p>
<p id="re"></p>