Why is RegExp replace() not working!?

function pageJump( obj )
  {
    var name    = obj.name;
    var val     = obj.options[obj.selectedIndex].value;
    var pattern = new RegExp(name+'=[0-9]+');
    var tmp     = location.href.split('?');
    var href    = tmp[0];
    var query   = tmp[1];
    if ( query && query.match(pattern) )
    {
      var replaceStr = name+'='+val;
      query.replace(pattern,replaceStr);
      
      alert(query+' : '+name+'='+val+' : '+pattern+' : '+replaceStr);
    }
    else if ( query && query != '' && query !== undefined )
    {
      query += '&'+name+'='+val;
    } else {
      query = name+'='+val;
    }
    parent.location.href = href+'?'+query;
  }

query = query.replace(pattern,replaceStr);

// was just... query.replace(pattern,replaceStr);