On Mar 7, 2012, at 6:54 AM, Markus Elze wrote:
Hello everybody,
this might be a trivial question, but I have been unable to find
this using Google. I am trying to replace double backslashes with
single backslashes using gsub.
Actually you don't have double backslashes in the argument you are
presenting to gsub. The string entered at the console as "C:\\" only
has a single backslash.
> nchar("C:\\")
[1] 3
There seems to be some unexpected behaviour with regards to the
replacement string "\\". The following example uses the string C:\\
which should be converted to C:\ .
> gsub("\\\\", "\\", "C:\\")
[1] "C:"
But I do not understand that returned value, either. I thought that
the 'repl' argument (which I think I have demonstrated is a single
backslash) would get put back in the returned value.
> gsub("\\\\", "Test", "C:\\")
[1] "C:Test"
> gsub("\\\\", "\\\\", "C:\\")
[1] "C:\\"
I thought the parsing rules for 'replacement' were different than the
rules for 'patt'. So I'm puzzled, too. Maybe something changed in 2.14?
> sub("\\\\", "\\", "C:\\", fixed=TRUE)
[1] "C:\\"
> sub("\\\\", "\\", "C:\\")
[1] "C:"
> sub("([\\])", "\\1", "C:\\")
[1] "C:\\"
The NEWS file does say that there is a new regular expression
implementation and that the help file for regex should be consulted.
And presumably we should study this:
http://laurikari.net/tre/documentation/regex-syntax/
In the 'replacement' argument, the "\\" is used to back-reference a
numbered sub-pattern, so perhaps "\\" is now getting handled as the
"null subpattern"? I don't see that mentioned in the regex help page,
but it is a big "page". I also didn't see "\\" referenced in the TRE
documentation, but then again I don't think that "\\" in console or
source() input is a double backslash. The TRE document says that "A \
cannot be the last character of an ERE." I cannot tell whether that
rule gets applied to the 'replacement'.
I have observed similar behaviour for fixed=TRUE and perl=TRUE. I
use R 2.14.1 64-bit on Windows 7.
--
David Winsemius, MD
West Hartford, CT
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