Re: Question aboug Regexp

From: Carsten Dominik <dominik@science.uva.nl>
To: Todd Neal <tolchz@tolchz.net>
Cc: emacs-orgmode@gnu.org
Subject: Re: Question aboug Regexp
Date: Tue, 23 May 2006 05:58:17 +0200	[thread overview]
Message-ID: <7f55ba6da92bc0c958dea27173f9e47b@science.uva.nl> (raw)
In-Reply-To: <87lkstxx24.fsf@tolchz.net>

On May 23, 2006, at 5:02, Todd Neal wrote:

>
>
> I am looking at why the following link does not work:
>
> [[elisp: (+ 1 2 3)]]
>
> I think that the problem lies with this regexp:
>
>      1	(defconst org-link-re-with-space2

The regexp org-link-re-with-space2 requires that the first character 
after elisp: is not a space character.  This was originally to make 
sure that the following would not be matched as a link:

I can explain you a feature in elisp: Parenthesis are everything.

This is not documented properly, thanks for reporting this.

In a regexp character class, the first character of a class is special 
and can be used to include character into he class which are otherwise 
difficult to get into a class, for example the minus "-" or a square 
bracket.  Since a character class [] or [^] is meaningless, this is a 
special case so that []] matches the closing bracket and [^]] 
everything besides the closing bracket.

- Carsten

 From the Emacs manual, node "Regexps", I have marked the important 
parts with "!" in the first column,

`[ ... ]'
      is a "character set", which begins with `[' and is terminated by
      `]'.  In the simplest case, the characters between the two
      brackets are what this set can match.

      Thus, `[ad]' matches either one `a' or one `d', and `[ad]*'
      matches any string composed of just `a's and `d's (including the
      empty string), from which it follows that `c[ad]*r' matches `cr',
      `car', `cdr', `caddaar', etc.

      You can also include character ranges in a character set, by
      writing the starting and ending characters with a `-' between
      them.  Thus, `[a-z]' matches any lower-case ASCII letter.  Ranges
      may be intermixed freely with individual characters, as in
      `[a-z$%.]', which matches any lower-case ASCII letter or `$', `%'
      or period.

      Note that the usual regexp special characters are not special
      inside a character set.  A completely different set of special
      characters exists inside character sets: `]', `-' and `^'.

!     To include a `]' in a character set, you must make it the first
!     character.  For example, `[]a]' matches `]' or `a'.  To include a
!     `-', write `-' as the first or last character of the set, or put
!     it after a range.  Thus, `[]-]' matches both `]' and `-'.

      To include `^' in a set, put it anywhere but at the beginning of
      the set.  (At the beginning, it complements the set--see below.)

      When you use a range in case-insensitive search, you should write
      both ends of the range in upper case, or both in lower case, or
      both should be non-letters.  The behavior of a mixed-case range
      such as `A-z' is somewhat ill-defined, and it may change in future
      Emacs versions.

`[^ ... ]'
      `[^' begins a "complemented character set", which matches any
      character except the ones specified.  Thus, `[^a-z0-9A-Z]' matches
      all characters _except_ ASCII letters and digits.

  !    `^' is not special in a character set unless it is the first
  !    character.  The character following the `^' is treated as if it
  !    were first (in other words, `-' and `]' are not special there).

      A complemented character set can match a newline, unless newline is
      mentioned as one of the characters not to match.  This is in
      contrast to the handling of regexps in programs such as `grep'.

>      2	  (concat
>      3	   "<?\\(" (mapconcat 'identity org-link-types "\\|") "\\):"
>      4	   "\\([^" org-non-link-chars " ]"
>      5	   "[^]\t\n\r]*"
>      6	   "[^" org-non-link-chars " ]\\)>?")
>      7	   "Matches a link with spaces, optional angular brackets 
> around it.")
>
>
> I am more used to PCRE so I may be incorrect, but is the "[^]" a typo?
>
>
> Also we have the following definition:
>
> (defconst org-non-link-chars "]\t\n\r<>")
>
>
> Doesn't this make line 4 evaluate to:
>
> "\\([^]\t\n\r<> ]"
>
> or is the right-bracket escaped somehow?
>
>
> Todd
>
>
>
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>
>

--
Carsten Dominik
Sterrenkundig Instituut "Anton Pannekoek"
Universiteit van Amsterdam
Kruislaan 403
NL-1098SJ Amsterdam
phone: +31 20 525 7477