Worth keeping in mind that you don't always want to split something into a separate function. When you split out a function, you should be asking yourself if you're actually simplifying something, or if you're just sweeping the complexity under the rug. If the person reading the first function will typically need to see the definition of the helper, the helper's definition is non-obvious, and the helper wouldn't be used elsewhere, it's probably not worth splitting out.

Another important consideration is whether or not the code block you split out makes any assumptions about the current state of the program (or object or whatever), and if those assumptions not being met would result in non-obvious bugs. (If this is true, you should probably wait until there's actual duplication somewhere before splitting the function out, IMO.) This is obviously not an issue with pure functions, but for most programs, most functions aren't pure.

See http://number-none.com/blow/john_carmack_on_inlined_code.htm... for further perspective on this.

Valid point. In this case a separate function would be better. It's not always an option though.

Edit: Rather I should say; It's not always an option that makes things better.

If you're writing a comment for a block of code is definitely a good time to think "Should this not be a separate function?" (I clearly need to do this more myself too.)

Besides unit testing, with regexes I always put a quick comment like "matches foo but not bar or baz" which might not be comprehensive but allows the next person to get a quick understanding of what the code does at a glance.