The O'Reilly book Mastering Regular Expressions maybe worth it. Old but regexes haven't changed much. [50-99] matches 5, any character in the range 0-9, or 9.

The O'Reilly book Mastering Regular Expressions maybe worth it. Old but regexes haven't changed much. [50-99] matches 5, any character in the range 0-9, or 9. So the 5 and second 9 are redundant.