Unit tests with a text fixture of every URL format you can think of, and every time one breaks your e.g. parser, you add it and some permutations to the list.

Unit tests with a text fixture of every URL format you can think of, and every time one breaks your e.g. parser, you add it and some permutations to the list. Eventually, you have a vast file with tons of nasty edge cases to ensure that your e.g. parser is robust and reliable.