The boilerplate on there is only a start. Any time you read or write a Unicode string, or do any manipulation of the string, you need to consider possible edge cases, and they can be nightmarish.
Consider something as simple as outputting zero padded numbers (so they form a nice column in the output), except that the numbers might not be Arabic, zero might not be 0, they might not be written in the direction you expect, and padding might not even make sense. This is how you go crazy.
Consider something as simple as outputting zero padded numbers (so they form a nice column in the output), except that the numbers might not be Arabic, zero might not be 0, they might not be written in the direction you expect, and padding might not even make sense. This is how you go crazy.