JavaScript判断输入内容是否合法的方法

示例代码

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML
1.0Transitional//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
<title>用户名是否可用</title>
<script type="text/JavaScript"language="javascript">
   function username(){
     var regex=/[a-zA-Z0-9_]*/;
     var username =document.getElementById('text').value;
     vara=regex.exec(username);
     if(a!=""){
       alert('用户名可用!');
     }else{
       alert('有非法字符');
     }
   }
</script>
</head>
<body>
<form>
   用户名：<input type="text"id="text" />
   <inputtype="button" onClick="username();" value="检验是否可用"/>
</form>
</body>
</html>