public void Validate()
{
if( _fromQuantity != null )
_fromQuantity.Validate();
if( _toQuantity != null )
_toQuantity.Validate();
if (_errorLimit!=null)
_errorLimit.Validate();
Regex regx = new Regex( PATTERN );
if( !regx.Match( ToString() ).Success )
throw new Exception(string.Format("Invalid Physical Expression - {0}", ToString()));
}