I'm not sure why you think it would be any easier to maintain that the non-regex solutions?

hdb's solution is extremely clever and obviously vastly superior in performance. Most impressive.

Nevertheless, the reasons I'd prefer to have to maintain the regex solution include:

• The algorithm find_substring() uses is extremely clever, perhaps even a little subtle. I always assume that's likely to be a disadvantage for future maintainability.
• And, indeed, after ten minutes of close inspection, I'm still not entirely sure I fully understand find_substring(). By Kernighan's Metric, if I'm not smart enough to understand it, I'm certainly not half smart enough to maintain it.
• Infinite loops and manually iterated string indexes always make me nervous. They are opportunities for off-by-one errors and overlooked edge-case behaviours to lurk...or to creep in when the code is subsequently updated.
• I genuinely prefer functional or declarative styles of programming. The regex solution describes exactly what it does (provided you're fluent in the regex dialect...which I am), whereas find_substring()'s implementation not at all self-explanatory (to me).
• Continuing on with the functional/imperative contrast: the regex solution uses exactly one automatically-preset read-only variable. In contrast find_substring() uses a couple of manually-assigned read-write variables. In my view that means the latter has several extra places where future well-intentioned modifications could quietly break something.
• I think that the regex solution would also be much easier to integrate into a larger parsing system, when the current simple text processing task subsequently grows more complicated (which it inevitably will).
• I can debug the behaviour of the regex-based solution visually using Regexp::Debugger. I'd have to use perl -d to debug find_substring(). <shudder>
• The find_substring() implementation somehow reminds me of my years of coding in C, and at this point I really don't need that kind of post-traumatic flashback undoing all the therapy. ;-)

Damian