Re: help with regexp function

[Stephen Berman]

On Wed, 22 Nov 2017 22:18:14 -0600 "B. T. Raven" <btraven@nihilo.net> wrote:
[...]
> On 11/22/2017 05:15, Stephen Berman wrote:
>> On Tue, 21 Nov 2017 17:30:15 -0600 "B. T. Raven" <btraven@nihilo.net> wrote:
>>
>>> Dear Emacs gurus:
>>>
>>> I can perform this inteactive substitution CM-%: \(^[0-9]+ \)\(.+\)
>>> -> \2 \1) in order to change a buffer line prefixed with a number
>>> into one post-fixed with the same number but I can't figue out how
>>> to do the same programatically to a whole region. I started with
>>> this code: [...]
>>
>> Here's a pretty direct translation of the interactive substitution:
> This works correctly but it isn't exactly what regexp-quote returns.

Note that (regexp-quote "\(^[0-9]+ \)\(.+\)") returns this string:
"(\\^\\[0-9]\\+ )(\\.\\+)", which matches only the literal string
"(^[0-9]+ )(.+)", so it's not what you want.

> Is there a function that produces "^\\([0-9]+\\) \\(.*\\)$" from
> "\(^[0-9]+ \)\(.+\)" ?

I don't know of any.  But in order for the Lisp reader to recognize a
backslash in a string as a backslash, you have to double it (because the
backslash is used as the escape character is the Lisp read syntax).  So
you can just write your regexp as you would when using
query-replace-regexp and then double each of the backslashes to use it
in a Lisp program.

> Did you replace .+ with .* just for greater generality?

Yep.

[...]
>> I can't tell what the problem without seeing the code that causes it.
>> Were you trying to treat the propertized string as a list because of
>> the #(...) notation?
>
> No, I don't understand that notation. I started with other regexp
> functions like query-replace-regexp but was getting type errors and
> general confusion.

I still can't tell how such errors arose, but it's probably not worth
pursuing now.

>>> ;; I have a function which is a black box to to me but it works in
>>> the larger context I have it in. Does match-string do something like
>>> this implicitly (casting a list as a string?)
>>
>> Not AFAIK.
>
> Here is the function I was talking about:
> (defun reverse-string (str)
>   (apply #'string (nreverse (string-to-list str))))
>
> It sounded like (append #'string ... was recasting a list to a string.
                   ^^^^^^
                   apply

I'm not sure it's helpful to think of it like that, since using a list
is an artefact of the function definition here: `string' takes one or
more characters but Emacs Lisp functions cannot return multiple values,
only single values such as a list of characters.  But you can dispense
with that intermediate step:

(defun reverse-string (str)
  (let ((l (length str))
        (nstr ""))
    (dotimes (i l nstr)
      (setq nstr (concat (string (aref str i)) nstr)))))

In fact, this is essentially how `nreverse' (and 'reverse') operate on
strings (so there's no need for `reverse-string').  In any case, I don't
see what this has to do with match-string.

Steve Berman