R help - Nov 2008 - Parsing regular expressions differently - feature request

Hi there,

I rejoiced when I realized that you can use Perl regex from within R. However, 
as the FAQ states "Some functions, particularly those involving regular 
expression matching, themselves use metacharacters, which may need to be 
escaped by the backslash mechanism. In those cases you may need a quadruple 
backslash to represent a single literal one. "

I was wondering if that is really necessary for perl=TRUE? wouldn't it be 
possible to parse a string differently in a regex context, e.g. automatically 
insert \\ for each \ , such that you can use the perl syntax directly? For 
example, if you want to input a newline as a character, you would use \n 
anyway. At the moment one says \\n to make it clear to R that you mean \n to 
make clear that you mean newline... this is pretty annoying. How likely is it 
that you want to pass a real newline character to PCRE directly?

If it's anyhow possible to pass everything between " and "
directly to PCRE
without expanding it internally in R, please add this to a future version (as 
an option like noescape=TRUE perhaps?)! I would love to use R instead of Perl 
for working with regex, without having to do two levels of escape all the 
time.

Thanks,
John

Some feature to simplify entry of backslashes has been mentioned many times
and keeps coming up from time to time. It would not only be useful for
regexp's
but also for latex and Windows path names and I too hope that it will be
addressed.

On Sat, Nov 8, 2008 at 7:20 AM, John Wiedenhoeft <john at nurfuerspam.de>
wrote:
> Hi there,
>
> I rejoiced when I realized that you can use Perl regex from within R.
However,
> as the FAQ states "Some functions, particularly those involving
regular
> expression matching, themselves use metacharacters, which may need to be
> escaped by the backslash mechanism. In those cases you may need a quadruple
> backslash to represent a single literal one. "
>
> I was wondering if that is really necessary for perl=TRUE? wouldn't it
be
> possible to parse a string differently in a regex context, e.g.
automatically
> insert \\ for each \ , such that you can use the perl syntax directly? For
> example, if you want to input a newline as a character, you would use \n
> anyway. At the moment one says \\n to make it clear to R that you mean \n
to
> make clear that you mean newline... this is pretty annoying. How likely is
it
> that you want to pass a real newline character to PCRE directly?
>
> If it's anyhow possible to pass everything between " and "
directly to PCRE
> without expanding it internally in R, please add this to a future version
(as
> an option like noescape=TRUE perhaps?)! I would love to use R instead of
Perl
> for working with regex, without having to do two levels of escape all the
> time.
>
> Thanks,
> John
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

On 08/11/2008 7:20 AM, John Wiedenhoeft wrote:
> Hi there,
> 
> I rejoiced when I realized that you can use Perl regex from within R.
However,
> as the FAQ states "Some functions, particularly those involving
regular
> expression matching, themselves use metacharacters, which may need to be 
> escaped by the backslash mechanism. In those cases you may need a quadruple
> backslash to represent a single literal one. "
> 
> I was wondering if that is really necessary for perl=TRUE? wouldn't it
be
> possible to parse a string differently in a regex context, e.g.
automatically
> insert \\ for each \ , such that you can use the perl syntax directly? For 
> example, if you want to input a newline as a character, you would use \n 
> anyway. At the moment one says \\n to make it clear to R that you mean \n
to
> make clear that you mean newline... this is pretty annoying. How likely is
it
> that you want to pass a real newline character to PCRE directly?
No, that's not possible.  At the level where the parsing takes place R 
has no idea of its eventual use, so it can't tell that some strings are 
going to be interpreted as Perl, and others not.

As Gabor mentioned, there have been various discussions of adding a new 
syntax for strings that are parsed literally, without processing any 
escapes, but no consensus on the right syntax to use.

There are currently some fragile tricks that let you avoid escapes, e.g. 
using scan() to read a line:

 > re <- scan(what="", n=1)
1: [^\\]
Read 1 item
 > re
[1] "[^\\\\]"

(I call this fragile because it works in scripts processed at console 
level, but not if you type the same thing into a function.)

So I agree, it would be nice to have new syntax to allow this.  Last 
time this came up, I argued for something like \verb in LaTeX where the 
delimiter could be specified differently in each use.  Duncan TL 
suggested triple quotes, as in Python.  I think now that triple quotes 
would be be better than the particular form I suggested.

Duncan Murdoch
> 
> If it's anyhow possible to pass everything between " and "
directly to PCRE
> without expanding it internally in R, please add this to a future version
(as
> an option like noescape=TRUE perhaps?)! I would love to use R instead of
Perl
> for working with regex, without having to do two levels of escape all the 
> time.
> 
> Thanks,
> John
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Duncan Murdoch murdoch at stats.uwo.ca Sat Nov 8 15:41:34 CET 2008
wrote:
> On 08/11/2008 7:20 AM, John Wiedenhoeft wrote:
> > Hi there,
> > 
> > I rejoiced when I realized that you can use Perl regex from within
R. However, 
> > as the FAQ states "Some functions, particularly those involving
regular 
> > expression matching, themselves use metacharacters, which may need
to be 
> > escaped by the backslash mechanism. In those cases you may need a
quadruple 
> > backslash to represent a single literal one. "
> > 
> > I was wondering if that is really necessary for perl=TRUE?
wouldn't
it be 
> > possible to parse a string differently in a regex context, e.g.
automatically 
> > insert \\ for each \ , such that you can use the perl syntax
directly? For 
> > example, if you want to input a newline as a character, you would
use \n 
> > anyway. At the moment one says \\n to make it clear to R that you
mean \n to 
> > make clear that you mean newline... this is pretty annoying. How
likely is it 
> > that you want to pass a real newline character to PCRE directly?
> 
> No, that's not possible.  At the level where the parsing takes place R
> has no idea of its eventual use, so it can't tell that some strings
are 
> going to be interpreted as Perl, and others not.
> 
> As Gabor mentioned, there have been various discussions of adding a
new 
> syntax for strings that are parsed literally, without processing any 
> escapes, but no consensus on the right syntax to use.
> ... [scan() example elided] ...
> So I agree, it would be nice to have new syntax to allow this.  Last 
> time this came up, I argued for something like \verb in LaTeX where
the 
> delimiter could be specified differently in each use.  Duncan TL 
> suggested triple quotes, as in Python.  I think now that triple quotes
> would be be better than the particular form I suggested.
>
> Duncan Murdoch
Would a string with this alternate quoting be tagged (e.g., with a class
that
inherits from character) so that the deparser could display it in the
style
in which it was input?  Functions which generate file names using the
native
Windows notation would like to have them displayed without the extra
backslashes.
However, adding a new class for this could mess up other things.
 
Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com