1. Error-checking - invalid regex, Out-of-bounds stuff. Do more testing.
2. More features - Non-greedy operators, backreferences, non-capturing groups, escaped characters inside character classes
3. Fix adding concatenation operators in shunting-yard function (very janky, compares against operators individually)
Ideas for flags:
    -m <num> : Print <num>th match (-m 1 = first match, -m 2 = second match)