参考:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head>
<body>
<script> 
function checkstr(str) 

num=str.length
var arr=str.match(/[^\\\\\\\\\\\\\\\\x00-\\\\\\\\\\\\\\\\x80]/ig)
if(arr!=null)num+=arr.length
alert(num)
} </script>
<input id="msg" value="blue蓝色"><input type=button onclick=checkstr(msg.value) value="检查">
</body>
</html>