On 08/11/2008 5:43 PM, Wacek Kusnierczyk wrote:
Duncan Murdoch wrote:
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.
well, it was not supposed to. it addresses the need for doubling
backslashes when a backslash character is an element of the regex.
\L could be an element of a regex in Perl.
Duncan Murdoch
foo = "foo\\n\n"
grep("\n", foo, perl=TRUE, value=TRUE)
mygrep("\n", foo, perl=TRUE, value=TRUE)
# both match the newline
grep("\\n", foo, perl=TRUE, value=TRUE)
mygrep("\\n", foo, perl=TRUE, value=TRUE)
# both match (guess what)
bar = "bar\n"
grep("\n", bar, perl=TRUE, value=TRUE)
mygrep("\n", bar, perl=TRUE, value=TRUE)
# both match the newline
grep("\\n", bar, perl=TRUE, value=TRUE)
mygrep("\\n", bar, perl=TRUE, value=TRUE)
# counterintuitively, grep matches (intuitively, it should match
backslash-n, not a newline, but there's just a newline in bar) -- i do
know why it matches, but i'm pretty sure for many of those who do it's
an inconvenient detail, and for those who don't it's a confusing annoyance
zee = "zee\\"
grep("\\", zee, perl=TRUE, value=TRUE)
mygrep("\\", zee, perl=TRUE, value=TRUE)
# grep fails, needs "\\\\"
conclusion? i'd opt for mygrep in my own code; i guessed this was what
john wanted, therefore the post.
vQ
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