I believe they're showing an example of where people would use (incorrect) regex patterns to validate. That's the whole thrust of the previous paragraph. `hadc0ffee` is not valid hex, but the function says it is due to, I presume, an incorrect regex pattern. I'm not familiar enough with regex to know why it's wrong though.

Edit: I was curious, so I looked through the linked docs for regex. They have the exact same pattern for checking hex in there as they do in this one. I guess it was just an error after all?