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Re: Fwd: How to check whether a character (or one-character string) is a

From: Marcin Borkowski
Subject: Re: Fwd: How to check whether a character (or one-character string) is a letter?
Date: Sun, 05 Oct 2014 02:11:03 +0200

On 2014-10-04, at 04:47, John Mastro wrote:

> [I first sent this directly to Marcin in error - yeah, I use the email 
> gateway]
> Hi Marcin,
> Marcin Borkowski <> wrote:
>> Assume that I have a character (taken from some string, which in turn is
>> copied from the buffer - so it need not be ASCII).  What is the best way
>> to check whether it is a letter within ASCII range?
>> The reason I'm asking is that I'm writing a function which converts an
>> arbitrary string to a valid (and nice) filename (e.g., only letters and
>> hyphens) - so basically I want to walk a string character by character
>> and convert any space to a hyphen and omit any other non-letter.  Am I
>> reinventing the wheel?
> There are a bunch of ways to do this, but one reasonable approach is to
> use a regular expression. I think this will do what you want:
>     (defun reasonable-filename (str)
>       (let* ((str (replace-regexp-in-string "[ \t\n\r]" "-" str))
>              (str (replace-regexp-in-string "[^a-zA-Z-]" "" str)))
>         str))

I think this is probably better than mapcar'ing through the string...

> This is a variation which will also allow the result to contain numbers:
>     (defun reasonable-filename (str)
>       (let* ((str (replace-regexp-in-string "[ \t\n\r]" "-" str))
>              (str (replace-regexp-in-string "[^a-zA-Z0-9-]" "" str)))
>         str))

This I don't want, since in case of equal filenames, I want to
differentiate them by appending a number, and allowing digits might
break this.  But thanks anyway.

> To answer your question about identifying whether a character is an
> ASCII letter, the key is that Emacs's characters are really "just"
> integers. Wikipedia has some charts[1] that show the numbers associated
> with the characters. The letters are conveniently grouped together, so
> we can use something like this:
>     (defun ascii-letter-p (char)
>       (and (characterp char)
>            (>= char 65)
>            (<= char 122)))
> (Of course, this only works if it's really a character, as opposed to a
> string of length one. If it's a string of length one you could either
> "extract" the character with `aref' or use a regular expression
> instead.)
> Hope that helps.

Yes it does!


Marcin Borkowski
Adam Mickiewicz University

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