Better question - why would you call an LLM (expensive in compute terms) for something that a regex can do (cheap in compute terms)

Regex is going to be something like 10,000 times quicker than the quickest LLM call, multiply that by billions of prompts

This is assuming the regex is doing a good job. It is not. Also you can embed a very tiny model if you really want to flag as many negatives as possible (I don't know anthropic's goal with this) - it would be quick and free.

I think it's a very reasonable tradeoff, getting 99% of true positives at the fraction of cost (both runtime and engineering).

Besides, they probably do a separate analysis on server side either way, so they can check a true positive to false positive ratio.