Javascript验证在Windows和Android手机上不起作用

我已经为名字"实现了Java脚本验证，该名字仅接受字符.它可以在计算机和Iphone上正常工作，但是如果我在其他手机上检查它，它也可以接受数字.我该怎么办?
我正在使用以下代码
在此先感谢

I have implemented java script validation for "first name" which accepts character only. It''s working proper on computer and Iphone but if I a checking It on other mobile It''s accept numbers also. What should I do for this problem?
I am using following code
Thanks In Advance

var alpha = "[ A-Za-z]";
var numeric = "[0-9]"; 
var alphanumeric = "[ A-Za-z0-9]"; 

function onKeyValidate(e,charVal){
    var keynum;
    var keyChars = /[\x00\x08]/;
    var validChars = new RegExp(charVal);
    if(window.event)
    {
        keynum = e.keyCode;
    }
    else if(e.which)
    {
        keynum = e.which;
    }
    var keychar = String.fromCharCode(keynum);
    if (!validChars.test(keychar) && !keyChars.test(keychar))   {
        return false
    } else{
        return keychar;
    }
} 




<input type="text" value="" name="first_name" id="first_name" placeholder="First Name" onkeypress="return onKeyValidate(event,alpha);"/>

我尝试过的事情:

我在上面的代码中尝试过此代码，请帮助我

What I have tried:

I tried this above code please help me