I guess because it's more comprehensive and declarative.

Because a String object can't be changed. In this code in the line "s+=string;" a new string is created every iteration.

Guys, one trick) Default constructor StringBuilder() constructs a string builder with no characters in it and an initial capacity of 16 characters. When those 16 characters are filled, sb automatically increases it's size and copies the data into it. So, if possible, it's better to define the expected size of your StringBuilder with this constructor StringBuilder(int capacity). So your code will be more effective)

One must be carefull with this method, because when you pass "int[] array", you pass a reference to the array. So, after the completion of the method the source array is also inverted.

else is not necessary here, i++ is enough)

And what if those three digits \d{3} start with 0, like 023? The number must be not valid in that case.

And what about ArrayIndexOutOfBoundsException?)

But you don't provide a check for not valid input in this code, so that's really not good practice)