This is an online tool that builds a JavaScript regular expression that matches characters that fall in any number of specified Unicode blocks.

Being bored, curious, and redundant all at the same time, I thought: "What if you want to check, in JavaScript, to see if a string is a JavaScript regular expression?" The obvious answer is, in a sense, self-referencing, because you'll have to check with regular expressions too. So basically, a regular expression to check regular expressions in JavaScript. Ugh.