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

[John Mastro <john.b.mastro@gmail.com>]

[I first sent this directly to Marcin in error - yeah, I use the email gateway]

Hi Marcin,

Marcin Borkowski <mbork@wmi.amu.edu.pl> 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))

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))

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.

[1] https://en.wikipedia.org/wiki/ASCII#ASCII_printable_code_chart

--
john