On 18/11/2008 1:36 PM, William Dunlap wrote:
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?
I don't recall anything like that in the proposal, but Duncan TL may
have had it in mind.
Duncan Murdoch
> 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
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