String(num) is actually more versatile and consistent than num.toString(). For example, toString() will throw an error if num is null or undefined, while String(null) safely returns "null". Even though it’s not needed here, using String() is more robust and still very readable.

Sorry, the rule is not clear.
Modified the description to:

Output: a compressed regular expression string. Note, if the string can not be compressed to a shorter string, returns the the original string.