I know of no Regex pattern that can handle all the old and new HTML as well as HTML5: believe me, as one who is looking to put HTML parser on FPGA/ASIC for higher speed, I've actually forayed down this rabbit hole a few times in the fruitless pursuit of identifying this elusive pure Regex pattern for HTML, et. al. Problem is in Regex's lack of support for multiple state machine and its needed interactions between the…
Meanwhile regular expression (the OG Regex) is just an NFA and should be easier to implement in circuit. The problem is an NFA circuit still needs exponential expansion (if minimized to DFA which is just power set of encoding and eliminating possible NFA states), and with Turing complete Regex you have halting problem -- both are hellish to solve unless P=NP