input: Write a python function to extract tags from an XML string.
  assert extract_tags('<root><tag>Python</tag><tag>JavaScript</tag></root>')==['Python', 'JavaScript']
  assert extract_tags('<root><tag>XML</tag><tag>JSON</tag><tag>HTML</tag></root>')==['XML', 'JSON', 'HTML']
  assert extract_tags('<root><tag>Data</tag><tag>Science</tag></root>')==['Data', 'Science']
output: def extract_tags(xml_string):
      import re
      return re.findall(r'<tag>(.*?)</tag>', xml_string)