Write a function expand$\_$string$($compressed$\_$str$)$ to expand a compressed string $($compressed$\_$str$)$ made up of characters followed by digits.

The compressed string consists of characters followed by digits, where each digit represents the number of times the preceding character should be repeated. For example, $"$a$3$b$2$c$1"$ should be expanded to "aaabbc".

Note:

The input string $($compressed$\_$str$)$ will consist of English alphabet letters $($case sensitive$)$ and digits.

A character will be followed by one or more digits.

The function should correctly expand the compressed string by repeating each character according to the digit$($s$)$ that follow it$.$

The function should return the expanded string.

It is assured that $2<=$ len$($compressed$\_$str$)<=1000.$