def validate_uri(value)
      URI.parse(value)
    rescue URI::InvalidURIError => e
      raise ValidationError, "invalid URI: #{e.message}"
    end
def validate_uri(value)
      URI.parse(value)
    rescue URI::InvalidURIError => e
      raise ValidationError, "invalid URI: #{e.message}"
    end
Validate URI string format following RFC 2396
 (see http://tools.ietf.org/html/rfc2396)