I am trying to develop a function to safely extract the Steem post_identifier from random strings (Steem transaction memos).
If the memo does not contain a post_identifier, the function should return None.

This is how far I got, before I found out, that d.tube does not use an @ before the author in their links:

def memo_to_post(memo):

    if not '@' in memo:

        return None

    reverse_memo = ''
    identifier = ''

    for char in reversed(memo):

        reverse_memo = reverse_memo + char

        if char == '@': 

            for char in reversed(reverse_memo):
    
                identifier = identifier + char

            break

    if identifier == '':

        return None

    return identifier

This is how beem tactles the issue:

def resolve_authorperm(identifier):
    """Correctly split a string containing an authorperm.
    Splits the string into author and permlink with the
    following separator: ``/``.
    """
    match = re.match("@?([\w\-\.]*)/([\w\-]*)", identifier)
    if hasattr(match, "group"):
        return match.group(1), match.group(2)
    match = re.match("([\w\-\.]+[^#?\s]+)/@?([\w\-\.]*)/([\w\-]*)", identifier)
    if not hasattr(match, "group"):
        raise ValueError("Invalid identifier")
    return match.group(2), match.group(3

With both methods, if someone was to send a d.tube link, it would not work correctly.

d.tube links look like this:

https://d.tube/#!/v/pottlund/m5cqkd1a

So, I have come up with this:

memos = [
         'bla.com/u/@user/identifier', 
         'bla.com', 
         'hello promo scam',
         '',
         'https://d.tube/#!/v/nowargraffitis/83wo23ut'
         ]

def memo_cutter(memo):

    reverse_memo = ''
    identifier = ''

    for char in reversed(memo):

        if char == '/' or char == '@': 

            break

        else:

            reverse_memo = reverse_memo + char
       
    for char in reversed(reverse_memo):

        identifier = identifier + char

    if identifier == '':

        identifier = None

    return identifier

def memo_to_identifier(memo):

    if memo == '':

        return None

    identifier = memo_cutter(memo)

    if identifier:

        author = memo_cutter(memo[:-(len(identifier) + 1 )])

    else:

        return None

    if not author:

        return None

    return (author +'/'+ identifier)
    

for memo in memos:

    identifier = memo_to_identifier(memo)

    print(str(identifier)) 

... which is kind of complicated,

but it works.

I am sure, there are much better ways to solve this question.
Probably with the split() function ...

Suggestions welcome !