I have played with this for a while, and here are some prompts that might be interesting.

Prompt: "URL regex"

  ^(https?:\/\/)?([\da-z\.-]+)\.([a-z\.]{2,6})([\/\w \.-]*)*\/?$

Prompt: "Email regex"

  /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))

This one's weirdly sophisticated for some reason.

Prompt: "An HTML tag with a close tag and attributes"

  <([a-zA-Z]+)[^>]*?(?<!\/)>.*?<\/\1>