I ran your code and it yields the error below. Tests have been updated since, do find a pattern in doing such exponentations.

TypeError: Cannot convert a BigInt value to a number
at Math.pow ()
at lastDigit (test.js:6:16)
at doTest (test.js:14:20)
at Context. (test.js:19:5)
at process.processImmediate (node:internal/timers:471:21)

Because your regex only matches when there is at least one letter after the apostroph.