Allow only Characters,Numbers and Special characters like - _

function ValidateUserName(object,e)//onkeyup

{

var Val=object.value;

var ValLen=object.value.length;

var allowChar=UpperChar+LowerChar+Numbers+"-_";

for (i=0; i<ValLen; i++)

{

if(allowChar.indexOf(Val.charAt(i)) == -1)

{

object.value=Val.substring(0,i);

break;

}

}

}

<input name="txtUser1" type="text" id="txtUser1"
onkeyup="javascript:ValidateUserName(this,event);" />