The Regex Tutorial Challenge

One part tutorial. One part challenge.

New to regular expressions? Rusty? Solve each level below to gradually (re)discover the perilous power of regex.

Try regex101.com for an absolutely wonderful regex testbed.
regular-expressions.info for a pretty nice regex explanation and tutorial.

Select a challenge level below. Consider the Test Cases for that level. Your goal is to make all the test cases pass. To do so, enter a regular expression and a substitution that transforms the left side of each test case to the expected output on the right.