import re


def extract_sentence_id(tag):
	"""
	Extract the sentence ID of current sentence.

	Args:
		tag (str): Sentence tag

	Returns:
		str: sentence ID
	"""
	if "<s" not in tag:
		return ""
	pattern = re.compile('id="[a-z0-9]+?"(?=\s)')
	res = re.findall(pattern, tag)
	if len(res) == 0:
		return None
	return res[0].replace('"', "").replace("id=", "")