[misc] Expression matching in calcurse: use regular expressions?


As a preliminary note, the way expression matching (for matching
dates, times, durations, etc) is currently implemented in calcurse is,
imho, mostly satisfying.

It might well be a case of "reinventing the wheel", but there is some
nice code in there; for instance, the DFA used by "parse_duration()"
in "src/utils.c" is implemented quite neatly.

However, I'm wondering if such an approach is viable. Maybe using a
proper regex library would be a good fit (I don't think it has been
discussed before).

Since POSIX requires some regex(7) primitives (see also regex(3)),
using these should not hamper calcurse's portability.

Here is the case for using regex(7) primitives where applicable,
instead of the current by-hand if-based (or DFA-based) parsing:

- allows to use fewer LoC for the same functionality, thus:
    - less time writing code in the first place (does not apply to
      existing code, however)
    - increased clarity: everything is expressing in terms of regexes
    - less time understanding code (from a calcurse hacker perspective)
    - less time modifying existing parsing code
    - fewer parsing bugs: if we use the right regex, then chances are
      that the parsing will be ok
- would not damage calcurse's portability (provided there aren't too
  much glibc-specific tricks in glibc's regex(3) primitives)

Here is the case against:
- it takes time to do
- it would be a shame to trash away neat code like parse_duration()'s DFA

What do you think? I know I'm probably going to get some "patches
welcome" answers; I'll try to put something together in the coming
days, but I would first like to have some opinions about this.

Anyway, this is non-critical for the 3.0 release, so there's probably
no need to hurry.


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