Re: [BUG] in org-property-drawer-re?

[Thorsten Jolitz <tjolitz@gmail.com>]

Carsten Dominik <carsten.dominik@gmail.com> writes:

> On 1.10.2013, at 19:50, Thorsten Jolitz <tjolitz@gmail.com> wrote:
>
>>
>> Hi List,
>>
>> for the navi-mode keyword-search for complete property drawers I copied
>>
>> ,-----------------------
>> | org-property-drawer-re
>> `-----------------------
>>
>> from org.el:
>>
>> #+begin_src emacs-lisp
>>  (concat "\\(" org-property-start-re "\\)[^\000]*\\("
>> 	  org-property-end-re "\\)\n?")
>> #+end_src
>>
>> #+results:
>> : \(^[ 	]*:PROPERTIES:[ 	]*$\)[^\\000]*\(^[ 	]*:END:[ 	]*$\)
>> : ?
>>
>> A bit unreadable, but you get the message ... here is my hopefully equivalent
>> version:
>>
>> ,--------------------------------------------------------------
>> | (:propertydrawer
>> |   . (concat "\\(^[\\s\\t]*:PROPERTIES:[\\s\\t]*$\\)[^\\000]*"
>> |             "\\(^[\\s\\t]*:END:[\\s\\t]*$\\)\\n?"))
>> `--------------------------------------------------------------
>>
>> But this did not match correctly in Bernt Hansens tutorial:
>
> Indeed, this is a bad regular expression, it is too greedy and will
> match all the way to the last :END: line it can find.  also, \\s is
> wrong, it should be just a space, so "[ \t]". Luckily
> this regular expression does not seem to be used in Org as far
> as I can see....


But, if this is equivalent to the #+results: block above, it is defined
in org.el without that one ? indicated that makes the difference:

,-----------------------------------------------------------
| (:propertydrawer
|   . (concat "\\(^[ \\t]*:PROPERTIES:[ \\t]*$\\)[^\\000]*?" <=
|             "\\(^[ \\t]*:END:[ \\t]*$\\)\\n?"))
`-----------------------------------------------------------

> Yes, you need the star to make it non-greedy.

the '?' I guess ...

> However, you can leave the \n after you have corrected the
> character class to "[ \t]" - it just means that
> the \n will be part of the match, but still allow
> for the possibility that the last line hits the end
> of the buffer.

ok, I see

> Ahhhh, regular expressions.  I think in my entire history
> as a programmer, learning about regular expressions was
> the biggest braintrip I ever had - still love them.

thanks god for M-x regexp-builder ;)

>> PS
>> Can anybody explain this marvelous construct in the regexp:
>>
>> ,---------
>> | [^\\000]
>> `---------
>
> This is just a cheep way to match any character at all, because \000 should
> not be part of any string (in C it indicates the end of a string).
> In principle you could put any character you are sure will not turn up,
> but \000 seems to be the safest choice.  It is
> faster (I think) than "\\(.\\|\n\\)*" because the first will
> just run fast and streight with a table lookup while the
> latter need to always alternate between two alternatives.
> I have not timed it, though.

This is a very nice trick, and the alternative looks easy too - I have
to remember this.

>> I often pondered about how to achieve its effect with other means, since
>> I did not find it in the Emacs Lisp manual.
>
> There you go - sometimes a brain is better than the Emacs manual :)

Thanks a lot

--
cheers,
Thorsten