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Re: Regular expression search

From: Kevin Rodgers
Subject: Re: Regular expression search
Date: Wed, 01 Nov 2006 13:04:11 -0700
User-agent: Thunderbird (Windows/20060909)

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 ()
  (let (limit-position)
    (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)))

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 ()
  (re-search-forward "[[:print:]]" (line-end-position)))


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