Re: Regular expression search

[Kevin Rodgers]

vb wrote:
> let's say I need a function to find first printable character on the line 
> where the pointer is. This is what I'm trying to use:
>
> (defun vb-first-printable ()
>   (interactive)
>   (let (limit-position)
>     (beginning-of-line)
>     (next-line 1)
>     (setq limit-position (point))
>     (previous-line 1)
>     (re-search-forward "\\S" limit-position 't)))
>
> when I try executing this, I get the following error:
>
> ====================================================
> Debugger entered--Lisp error: (invalid-regexp "Premature end of regular 
> expression")
>   re-search-forward("\\S" 3543 t)
>   (let (limit-position) (beginning-of-line) (next-line 1) (setq limit-position 
> (point)) (previous-line 1) (re-search-forward "\\S" limit-position (quote 
> t)))
>   vb-first-printable()
>   call-interactively(vb-first-printable)
> ===================================================
>
> the same problem happens when I try re-search-forward from the command line: 
> if I enter "\S" as the pattern to search, I get "premature end of regular 
> expression" error,  but if I enter "\\S" as the regular expression pattern, 
> the only thing it finds is this pattern (\\S) itself (as I try it on the same 
> file where the source code is).
>
> What am I missing here?

Commands that prompt you for a regexp allow you to enter it directly;
but when calling a Lisp function you have to specify the regexp as a
string, and in order to represent a backslash within a (double quote- 
delimited) string literal you must double it: "This string has 1 
backslash (here: \\) and 1 double quote (here: \")."  And of course
the `\\' regexp matches the backslash character itself.

The manual states:

,----
| `\sC'
|      matches any character whose syntax is C.  Here C is a character
|      that designates a particular syntax class: thus, `w' for word
|      constituent, `-' or ` ' for whitespace, `.' for ordinary
|      punctuation, etc.  *Note Syntax::.
|
| `\SC'
|      matches any character whose syntax is not C.
`----

So you must specify a syntax class `C', e.g. `w' for word constituent,
`-' or ` ' for whitespace, `.' for ordinary punctuation, etc.

But as there is no syntax class for printable or non-printable
characters, that seems like a dead end.  But there is the [:print:]
character class that you can use in regular expressions.

And finally, all that limit-position/next-line/point/previous-line stuff
can be replaced by line-end-position:

(defun vb-first-printable ()
  (interactive)
  (beginning-of-line)
  (re-search-forward "[[:print:]]" (line-end-position)))

--
Kevin