# [R] Parsing regular expressions differently - feature request

William Dunlap wdunlap at tibco.com
Tue Nov 18 19:36:11 CET 2008

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
> > 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