On 08/11/2008 3:16 PM, Wacek Kusnierczyk wrote:
Duncan Murdoch wrote:
On 08/11/2008 11:03 AM, Gabor Grothendieck wrote:
On Sat, Nov 8, 2008 at 9:41 AM, Duncan Murdoch <[EMAIL PROTECTED]>
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.
Here's a quick hack to achieve the impossible:
That might solve John's problem, but I doubt it. As far as I can see it
won't handle \L, for example.
Duncan Murdoch
mygrep = function(pattern, text, perl=FALSE, ...) {
if (perl) pattern = gsub("\\\\", "\\\\\\\\", pattern)
grep(pattern, text, perl=perl, ...)
}
(text = "lemme \\ it")
# [1] "lemme \\ it"
nchar(text)
# [1] 10
(pattern = "\\")
# [1] "\\"
nchar(pattern)
# [1] 1
grep(pattern, text, perl=TRUE)
# can't go, impossible!
mygrep(pattern, text, perl=TRUE, value=TRUE)
# [1] "lemme \\ it"
vQ
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