Retrieves the id for an etext.
	Args:
		lines (iter): The lines of the etext to search.
	Returns:
		int: The id of the etext.
	Raises:
		ValueError: If no etext id was found.
	Examples:
		>>> etextno(['Release Date: March 17, 2004 [EBook #11609]'])
		11609
		>>> etextno(['Release Date: July, 2003 [Etext# 4263]'])
		4263
		>>> etextno(['Release Date: November 29, 2003 [Eook #10335]'])
		10335
		>>> etextno(['December, 1998  [Etext 1576#]'])
		1576
		>>> etextno(['Some lines', 'without', 'Any [Etext] Number'])
		Traceback (most recent call last):
			...
		ValueError: no etext-id found

def etextno(lines):
	"""Retrieves the id for an etext.
	Args:
		lines (iter): The lines of the etext to search.
	Returns:
		int: The id of the etext.
	Raises:
		ValueError: If no etext id was found.
	Examples:
		>>> etextno(['Release Date: March 17, 2004 [EBook #11609]'])
		11609
		>>> etextno(['Release Date: July, 2003 [Etext# 4263]'])
		4263
		>>> etextno(['Release Date: November 29, 2003 [Eook #10335]'])
		10335
		>>> etextno(['December, 1998  [Etext 1576#]'])
		1576
		>>> etextno(['Some lines', 'without', 'Any [Etext] Number'])
		Traceback (most recent call last):
			...
		ValueError: no etext-id found
	"""
	for line in lines:
		match = ETEXTRE.search(line)
		if match is not None:
			front_match = match.group('etextid_front')
			back_match = match.group('etextid_back')
			if front_match is not None:
				return int(front_match)
			elif back_match is not None:
				return int(back_match)
			else:
				raise ValueError('no regex match (this should never happen')
	raise ValueError('no etext-id found')