function clear(str) { 
str = str.replace(/,/g, "");//取消字符串中出现的所有逗号 
return str; 
} 

if(isNaN(document.login.imgcode.value)){ 
alert('验证码必须是数字！') 
document.login.imgcode.focus(); 
return false; 
} 

<script> 
String.prototype.replaceAll=function (AFindText,ARepText){ 
raRegExp=new RegExp(AFindText,"g");returnthis.replace(raRegExp,ARepText); 
} 
alert("axxxbxxxxcxxxxx".replaceAll("x","_"))</script>

var a="今天是星期五， 明天又可以放假了&好好休|息一下"; 
var converter = document.createElement("DIV"); 
converter.innerHTML = a; 
var b = converter.innerText; 
converter = null; 
var c=b.replace(/[&\|\\\*^%$#@\-]/g,"");
alert(c);