input: Write a function to create a regular expression in Python that matches all numeric strings. The function should return the pattern as a string.
  assert create_numeric_regex()=='^[0-9]+$'
  assert re.match(create_numeric_regex(), '123456789')
  assert not re.match(create_numeric_regex(), 'abc123')
  assert re.match(create_numeric_regex(), '0')
  assert not re.match(create_numeric_regex(), '')
output: def create_numeric_regex():
      return '^[0-9]+$'