From: Thorsten Jolitz <tjolitz@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Possible to write 'beginning of line' in a String?
Date: Wed, 03 Apr 2013 09:06:27 +0200 [thread overview]
Message-ID: <87y5d0nle4.fsf@gmail.com> (raw)
In-Reply-To: 87k3okipwg.fsf@kuiper.lan.informatimago.com
"Pascal J. Bourguignon" <pjb@informatimago.com> writes:
>> Thats not what I mean. I set a variable with string values that are
>> inserted by other libraries (not under my control), but I want to use
>> these string values for regexp searches in my own program too.
>
> No.
>
> Regular expressions are different from the string they match in
> general.
>
> Literal regular expressions are of course identical to the string they
> match. (string-match "abc" "abc") --> 0 ; is true
>
> Some special cases may also be identical. For example, "[][*]*" matches
> itself. (string-match "[][*]*" "[][*]*") --> 0
>
> But not in general, and not in the case of matching the beginning of the
> line, since this matches 0 characters, but a position, while obviously
> the regular expression for it is 1 or more characters.
>
> One easy solution is to duplicate your variables:
>
> (defvar *thingy-string* "hello")
> (defvar *thingy-regexp* "\\`hello")
>
>
> Another solution would be to write a function that would generate from a
> regular expression a minimal string that would match. But in general,
> that would be you'd have to re-implement the full regexp parser, since
> AFAIK, emacs doesn't have a public API to map regexp strings to regexp
> sexps that can easily be processed.
>
>
> Notice the difference between ^ which means beginning of string, and \\`
> which means beginning of line:
>
> (string-match "^hello" "hello") --> 0
> (string-match "\\`hello" "hello") --> 0
>
> (string-match "^world" "hello\nworld") --> 6
> (string-match "\\`world" "hello\nworld") --> nil
Actually I kind of overlooked "\\`" until now and always used "^", but
in this special case my question refers to it would not make a
difference.
>> But thanks to everyone, I was probably looking for an (obscure magic)
>> non-printable control-character that functions in a string like "^" does
>> in a regexp, but the way to handle this is probably 'take the string and
>> add "^" in front of it before using it as regexp'.
>
> There's no such thing as a control-character. There are characters.
> And there are encodings, which map characters to codes. Then in a given
> encoding, there may be codes that don't correspond to any characters
> that may be used to "control" or some other purpose, that one could call
> control codes.
>
> Don't let emacs confuse you by the fact that it uses integers to
> represent characters. It still decode and encode them to sequences of
> code when doing I/O.
I simply refered to the common use of the term 'ASCII control
characters', but thanks for the insights about what is going on under
the hood in Emacs.
--
cheers,
Thorsten
prev parent reply other threads:[~2013-04-03 7:06 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-04-02 7:26 Possible to write 'beginning of line' in a String? Thorsten Jolitz
2013-04-02 8:11 ` Teemu Likonen
2013-04-02 9:22 ` Thorsten Jolitz
2013-04-02 8:16 ` Andreas Röhler
2013-04-02 9:20 ` Thorsten Jolitz
2013-04-02 9:24 ` Óscar Fuentes
2013-04-02 9:41 ` Andreas Röhler
2013-04-02 9:58 ` Thorsten Jolitz
[not found] ` <mailman.23288.1364897437.855.help-gnu-emacs@gnu.org>
2013-04-02 21:27 ` Pascal J. Bourguignon
2013-04-03 7:06 ` Thorsten Jolitz [this message]
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