def METHOD_NAME(input: str) -> list[str]:
    """
    Takes a string, representing a regular expression as input, and tokenizes
    it.
    :param input: string, representing a regular expression.
    :returns: List of tokens.
    """
    # 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