Python Regex: how to parse repeated groups in a string?

Question

I want to match a pattern with a string including pure numbers, such as '2324235235980980' with a pattern like as described below:

The pattern is '2-6-8-7-4', in which the pattern starts with 2, transit to 6, either self-loop at 6 or transit to 8, then it could go back and forth between 6 and 8, could self-loop at 8, or could transit to 7. And the same thing for 7. One more thing for 7 is 7-8-6-8-7 could happen. Finally, 7 could reach 4, once it reaches 4, the pattern is done. During the process, if it reaches out to other points, then it has to start with 2 again to be counted. I use

import re    
re.findall(r'(2((6+8+)+)7)', test_string)

the output includes '2666686888668887', but when I add 4, I don't know the syntax to compile this. Has anyone an idea? Thanks a lot!

Answer 1

I think this is easier achieved than initially expected:

26[68]+?[687]+?4

2-followed-by-6-followed-by-6|8-followed-by-6|8|7-followed-by-4.

The only not so obvious part is to make the pattern lazy.

Here is an even better pattern:

\b26?([^7]6|8|[^6]7)+?4\b

2-followed-by-(not7)6|8|(not6)7-followed-by-4.