Answer: What can regular expressions NOT do?
The question is not so much 'What Can't Regexp Do?', but when shouldn't you use it.
There are times when processing strings where it is easier and faster to use a substr
to grab what you need from input. This is especially true if you know that the length
of the input elements does not change (ie a fixed-width flat text file...). I guess the
analogy would be that if you wanted to dig postholes for a fence, you could use a shovel,
which is powerful and has many different ways to use, but is not as efficient as a posthole
digger. Don't get me wrong, I love regexps, but sometimes, they are not the best tool for
the job. :)