Rearrange and improve comments
Changing "PCRE" references to "regex" comments

The latter is a better-known term than the former.
Fix a bug in array shifting

We were using array_unshift on an associative array, which doesn't
work, because the key doesn't get pushed onto the stack. Instead, we
need to engage in a bit of hackery.
Store a list of prefixes, in order

This is a step towards solving issue #1, which requires stateful
knowledge of the ordinal values of prefixes.
Support the provision of utterly unstructured text

Previously, this required that the text already be broken up into
per-paragraph (or, better, per-section) units. Now this tries to do so
itself, if the text hasn't been prepared properly.
Make prefix examples available within the matching loop
Determine whether we're starting a list of Roman numerals

Per #1. This is a lazy solution, but it's also very achievable, so it
has that going for it.
Terminate with a newline