input: Write a python function to extract the version string from a file using regular expressions.
  assert extract_version("__version__ = '1.0.0'") == '1.0.0'
  assert extract_version("__version__ = \"2.1.3\"") == '2.1.3'
  assert extract_version("some random text\n__version__ = '3.4.5'\nmore random text") == '3.4.5'
output: def extract_version(file_content):
      import re
      match = re.search(r"__version__ = ['\"]([^'\"]+)['\"]", file_content)
      return match.group(1) if match else None