Regex Ethereum address

An Ethereum address isn't just "0x + 40 hex characters": the EIP-55 standard encodes a checksum directly into letter casing, computed from a Keccak-256 hash. A GitHub gist perfectly illustrates the limit of a plain regex: it compares two identical addresses except for a single case change - one with a valid checksum, one invalid - and both pass the exact same regex test identically. None of the three attempts compared here compute that checksum; it's structurally not possible with a regex alone.