Re: PATCH: New options for the PCRE module

[Phil Pennock <zsh-workers+phil.pennock@xxxxxxxxxxxx>]

Chiming in because I'm the most recent person to mess with regex
functionality (I think).

On 2009-02-26 at 23:15 -0800, Jon Strait wrote:
> 1.  A new '-s' option to pcre_compile.  This is the frequently set 
> PCRE_DOTALL option, allowing the dot character to match a newline as well.

This makes sense, since we already have the other common options as
flags.  In the meantime, you know about the internal option setting
feature of PCRE syntax, right?  Putting (?s) at the start of the pattern
is equivalent.

> On the safe side, regarding the possibility of multi-byte characters, 
> I'm assuming that the returned offset positions are only for sending 
> back to pcre_match and not for indexing on a match string, because the 
> offsets are in byte count, not character count.

This is dubious.  I can see someone quite reasonably using
$var[start,end] for substring extraction; the shell should be internally
consistent.  In a worst-case scenario, there could be another option to
select which offset semantics shall be used.  Peter's work on UTF-8
support has so far managed to keep the user from ever knowing or caring
about this.

Or return four numbers instead of two, so that anyone using the
interface has to be aware of the difference and can think about it.

I'm not coming up with a more elegant solution.

> 3.  A needed correction: all of the module's external variables are now 
> unset on each match attempt, so that a failed match will be obvious.

Well, the exit status is set already.  And since the last shell release,
we've documented explicitly that nothing is altered:

2009-01-15:
        * 26312: Phil Pennock: Doc/Zsh/cond.yo, Doc/Zsh/mod_pcre.yo,
        Doc/Zsh/mod_regex.yo: Document no variables altered on failed
        match.

On the other hand, there's value to a reset too.  I don't have a strong
preference either way, but now is the time to fix it, before there's
been a release which documents the behaviour.  :)  Part of the problem
is that pcre_regex has been in Zsh for many years and we tend to be
cautious when changing behaviour.

I doubt that anyone is relying on the value being unchanged after a
match attempt.  Thus the lack of a strong preference.  Anyone else?

While you're at it, there's also the zsh/regex module which uses the
system's normal extended regex libraries and if you're changing the
semantics of one, both should change.

> Could someone please point me to the doc files that would need updating 
> (for the zshmodule man page), or if someone here has that part 
> automated, I can send them whatever targeted write-up they want.

Doc/Zsh/mod_pcre.yo (and mod_regex.yo), which are in YODL format.

> -    ret = pcre_exec(pcre_pattern, pcre_hints, *args, strlen(*args), 0, 0, ovec, ovecsize);
> +    ret = pcre_exec(pcre_pattern, pcre_hints, *args, strlen(*args), offset_start, 0, ovec, ovecsize);

How gracefully does pcre_exec() fail when offset_start is set to a value
larger than the length of the string?  To maxint-smallnumber?

-Phil