This article is a continual WIP, where I will add particular things of note when coming up with solutions to the problems.
The importance of sample cases
Small optimizations can sometimes break the logic of your code - so rather than waiting for a long run to finish, make sure you run your test cases first so you can fail early and fix it fast.
Regex
Referring back to a group matched in regex again with \1 (number corresponding with the captured group) helped detect recurring patterns
Partial capture {0,2} Allowed me to factor in recurring patterns that may clip at the end, e.g.
0.123456123456123
Approaches
Sometimes the ‘ugly’ way is the fastest A lot of the problems use repeated functions, like isPrime, isPandigital. Because these can be costly enough, it’s probably worthwhile just generating a file with valid combinations, loading them in at the start and searching for them, rather than do the math each time.