If you want to build a regular expressions parser as a learning exercise, you might want to look into automatons. Lots of easy to grasp (and best of all: visual) concepts to learn there.
Second that. If you are going to go down this route, I wrote a simple tutorial with lots of visuals to construct a regular expression from a finite state automaton using JFLAP ( jflap.org ), for the "inverse fizzbuzz" problem -
http://www.jasq.org/2/post/2012/06/fbfbfbfbfffbfbffbffbfbffb...
My original automaton only has 8 states. ( http://www.jasq.org/uploads/1/0/5/6/10564637/977128_orig.png ) I guess that's the price you pay for cycles. ( Mine is a cyclic graph, but Rex produces a directed acyclic graph. Since they both must capture the same regex, one of them would be much larger than the other )
I wrote a detailed set of articles for GameDev back at the time (2004) that goes the whole way - full regex matcher (for simple regexes) including NFA/DFA construction.
You also probably have better things to do with your time than writing your own webserver, or creating your own language and compiler, or creating a database. But every time you do one of those things, you learn a lot about the things you actually use, and you end up being a far, far better programmer on the whole. If you only do the things you need to do, you're never going to be a great programmer.
My point was that the original post seemed really judgmental in implying that building a RegEx engine ought to be done by everybody. Instead of "every programmer ought to have the experience", they could have said, "it can be valuable to have the experience".
But a blanket statement that every programmer should do X... is really annoying and shows a very narrow worldview. Programmers are diverse, and do lots of different things.
Wow, overreaction... there is a standard human communication idiom that is being followed here. For example movies are labeled as "must see", food is described as "you have to try...", and so on. Everyone[1] understands this to be a hyperbolic phrasing that isn't to be taken literally, but to really emphasize how important the speaker[2] finds the experience. Relax a little on the pedantry, it invalidates any statements made about better uses of one's time.
[1] The sentence that references this footnote uses the phenomenon it describes for emphasis.
[2] This is a similar, but distinct phenomenon of using a term, in this case speaker, with the implicit understanding that it can be generalized, in this case to "communicator". It shouldn't be confused with the situation described, but is also a useful pattern to understand for basic communication.
Yeah, but writing your own engine will help you understand why your innocent-looking expression has stupid exponential performance. (Assuming you implement something akin to Perl-style regexps that use backtracking rather than actually regular expressions using an NFA.)
It will also help you understand why engines like re2[1] are absurdly faster than normal engines.
