Re: Possible to write 'beginning of line' in a String?

[Thorsten Jolitz]

"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