1. Error-checking - invalid regex, Out-of-bounds stuff. Do more testing.
2. More operators - numeric specifier, Escaped characters, anchors, word boundaries, Inverting match inside character class
    a. Make the zero-length checking more rigorous - a? does not match empty string, even though it should.
3. Add support for zero-length matches - see https://regex101.com/r/8Ec4ue/1
4. Fix adding concatenation operators in shunting-yard function (very janky, compares against operators individually)