only allow English characters and numbers for text input

Assuming you also want to accept spaces:

$("#user").keypress(function(event){
    var ew = event.which;
    if(ew == 32)
        return true;
    if(48 <= ew && ew <= 57)
        return true;
    if(65 <= ew && ew <= 90)
        return true;
    if(97 <= ew && ew <= 122)
        return true;
    return false;
});

If you don't want to accept spaces then remove the if(ew == 32) return true;

JSFiddle

<input type="text" id="firstName"  onkeypress="return (event.charCode >= 65 && event.charCode <= 90) || (event.charCode >= 97 && event.charCode <= 122) || (event.charCode >= 48 && event.charCode <= 57)" />

The ASCII Character Set : https://www.w3schools.com/charsets/ref_html_ascii.asp