I frequently find myself trying to quickly explain the basics of JavaScript regexp, and doing a hasty or incomplete job. Instead, I'm making this. It is by no means a complete reference. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp for that.
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- any character[abc]
- any character that matches one of the characters inside the brackets (in this case "a", "b", or "c")