ChatGPT wrote some unit tests and found a small bug.

When asked to fix the bug, it came up with a different regex, which
would completely change what's valid and what's not, so that's totally
wrong:

    /^\s*(-)?([0-9]+)(?:[,.]([0-9]{1,2}))?\s*$/

When asked to fix it in another way, without changing the regex, it
suggested stripping the sign completely, which is even more wrong.

So I fixed it myself :)

lib/RevBank/Amount.pm

@@ -81,6 +81,8 @@ sub parse_string($class, $str) {
         = $str =~ /^\s*(?:\+|(-)?)([0-9]+)?(?:[,.]([0-9]{1,2}))?\s*$/
         or return undef;
 
+    defined $int or defined $cents or return undef;
+
     $int //= 0;
     $cents //= 0;
     $cents *= 10 if length($cents) == 1;  # 4.2 -> 4.20