前缀输入值

I am creating a form and dynamically populating a field using php so the field value looks like this 01-02-2015-01. I need to prefix the value with a number depending on the value of a select box. So if ( condition ) the prefix will look be 888 so the input value is now 888-01-02-2015-01. I need to do this dynamically so javascript/jQuery is what I need to use. Any help out be appreciated.

Untested code, but probably should work

Html :

<select id='currency'>
  <option val='$'>$</option>
  <option val='Rs.'>Rs.</option>
</select>
Amount : <input type='text' value='1200' id='amount'/>

Jquery :

  $(function(){
     $("#currency").change(function(){
       var symbol = $(this).val();
       var currentValue = $('#amount').val();
       $.each(['$','Rs.'],function(i, a){
         if(currentValue.indexOf(a)==0){  //if the currentValue starts with any matching symbol, then remove it.
           currentValue = currentValue.replace(a,'');
         }
       });
       $("#amount").val(symbol + ' ' + currentValue);
     });

    //In order to fire change manually, do this
    $("#currency").change();
});