Allow Only Upper Char, Lower Char and &-_

function OnlyCharactersWithAmpersand(object,e)//onkeyup

{

var Val=object.value;

var ValLen=object.value.length;

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

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

{

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

{

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

break;

}

}

}

<input name="txtAmbersand1" type="text" id="txtAmbersand1" onkeyup="javascript: OnlyCharactersWithAmpersand (this,event);" />