How to prevent non-alphanumeric input in javascript?

Question

$(\"#user\").keyup(function(e){ 
    var regx = /^[A-Za-z0-9]+$/;
    if (!regx.test(\'#user\')) 
    {$(\"#infoUser\").html(\"Alphanumeric only allowed !\");}
);}
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Answer 1

change:

if (!regx.test('#user')) 

to

if (!regx.test( $(this).val() ) ) 

Do:

$("#user").keyup(function(e){     
    var str = $.trim( $(this).val() );
    if( str != "" ) {
      var regx = /^[A-Za-z0-9]+$/;
      if (!regx.test(str)) {
        $("#infoUser").html("Alphanumeric only allowed !");
      }
    }
    else {
       //empty value -- do something here
    }
});

JS Fiddle example