code: def tokenize_regex(input):
      
      # Regular expression for tokenizing other regular expressions.
      p = re.compile(r'''^(
          \(\?P\<[a-zA-Z0-9_-]+\>  | # Start of named group.
          \(\?#[^)]*\)             | # Comment
          \(\?=                    | # Start of lookahead assertion
          \(\?!                    | # Start of negative lookahead assertion
          \(\?<=                   | # If preceded by.
          \(\?<                    | # If not preceded by.
          \(?:                     | # Start of group. (non capturing.)
          \(                       | # Start of group.
          \(?[iLmsux]              | # Flags.
          \(?P=[a-zA-Z]+\)         | # Back reference to named group
          \)                       | # End of group.
          \{[^{}]*\}               | # Repetition
          \*\? | \+\? | \?\?\      | # Non greedy repetition.
          \* | \+ | \?             | # Repetition
          \#.*\n                   | # Comment
          \\. |
  
          # Character group.
          \[
              ( [^\]\\]  |  \\.)*
          \]                  |
  
          [^(){}]             |
          .
      )''', re.VERBOSE)
  
      tokens = []
  
      while input:
          m = p.match(input)
          if m:
              token, input = input[:m.end()], input[m.end():]
              if not token.isspace():
                  tokens.append(token)
          else:
              raise Exception('Could not tokenize input regex.')
  
      return tokens
docs: Takes a string, representing a regular expression as input, and tokenizes
      it.
  
      :param input: string, representing a regular expression.
      :returns: List of tokens.