docs: def _capitalize_first_letter(word):
      """Return a regex pattern with the first letter.
  
      Accepts both lowercase and uppercase.
      """
      if word[0].isalpha():
          # These two cases are necessary in order to get a regex pattern
          # starting with '[xX]' and not '[Xx]'. This allows to check for
          # colliding regex afterwards.
          if word[0].isupper():
              return "[" + word[0].swapcase() + word[0] + "]" + word[1:]
          else:
              return "[" + word[0] + word[0].swapcase() + "]" + word[1:]
      return word