对于此特定示例，
var thenum = thestring.replace( /^D+/g, ''); // replace all leading non-digits with nothing
在一般情况下：
thenum = "foo3bar5".match(/d+/)[0] // "3"
由于这个答案由于某种原因而受到欢迎，因此有一个好处：正则表达式生成器。
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: <pre>' + re + '</pre>';}<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>
