On: RegExr: Learn, Build, and Test RegEx

[dang]

My favorite of these, because it's the simplest:

"As an aside, it's easy to test your understanding of regular expressions interactively: the program

  $1 ~ $2

lets you type in a string and a regular expression; it echoes the line back if the string matches the regular expression."

From the classic book "The Awk Programming Language" (https://news.ycombinator.com/item?id=13451454).