I got the feeling that a lot of those »I have to do this in a single regex« questions come from places where a single regex is basically the only API you have available. Something like form input validation where the framework provides a handy regex it uses for validation, but doesn't expose actual validation callbacks or events do to the same in code without having to redo everything around it. It's only a hunch, but when I have the opportunity to use code to validate a string I probably wouldn't assume that code to be a mandatory one-liner, even as a beginner developer.
