input: Write a function to identify a string containing an 'a' followed by at most one 'b'.
  assert text_match("ac")==('Found a match!')
  assert text_match("dc")==('Not matched!')
  assert text_match("abbbba")==('Found a match!')
output: def text_match(text):
      import re
      if re.search(r'a[b]?[^b]*', text):
          return 'Found a match!'
      else:
          return 'Not matched!'