You could create a function that generates all valid smileys. This is usable, but if the number of valid variations exceeds 10, this becomes difficult to maintain or check.

Importing inside a function is frowned upon.
Compilation of a regex is useful if you re-use it. Either define it outside the function (for reuse) or don't bother. Also, when you compile, you can use smiley.match(el).

Quite surprising this wasn't a requirement or tested. It's bad practice to mutate something inside a function when it is not explicitely expected.

Most (if not all) solution I looked through convert the number to a string. I wanted to find a method that did not require converting a number to text, i.e. a more 'pure math' solution.

This code finds the number of digits (power) and finds each digit. To get the second digit from 371 it finds the remainder 371 % 100 = 71, divides by 10 and truncates (int(71 / 10)).