Facebook Messenger is blocking me from sending any string containing the attacker's wallet address bc1qxy2kgdygjrsqtzq2n0yrf2493p83kkfjhx0wlh. I'm trying to send blockchain explorer URL's to friends and it's failing.

Facebook messenger has extremely sensitive filters. It seems to block most links I try to send. Most recently, I tried to send a GIF of a beautiful city on a river to my girlfriend. Nope, blocked. The link was a GIF sharing site, I don't remember which one, but it's highly unlikely there's any malware on it so it must be a porn filter. And as for sending links to my staging site to a couple of tech friends... forget about it.

How would you write a regex that does this? How do you determine whether a string of characters is random?

The classic way to do it is see how well it compresses, though that requires a certain minimum length.

You can probably calculate the entropy / randomness of a string via a fairly simple algorithm.

Reminds me of a thing years ago where a virus used a Twitter account's messages as its command & control system. Said twitter account encoded the commands in base64 I believe.