> -----Original Message-----
> From: r-devel-boun...@r-project.org
> [mailto:r-devel-boun...@r-project.org] On Behalf Of amira...@uwo.ca
> Sent: Monday, January 19, 2009 10:25 AM
> To: r-de...@stat.math.ethz.ch
> Cc: r-b...@r-project.org
> Subject: [Rd] sub and gsub treat \\ incorrectly (PR#13454)
>
> Sub and gsub treat \\ replacement pattern incorrectly
>
> I expect
> sub("a","\\", "a", perl=T)
> to produce
> [1] "\"
> instead it generates
> [1] ""
>
> On the other hand, if I run
> sub("a","\\\\", "a", perl=T)
> it correctly outputs
> [1] "\\"
The replacement pattern may include \\digit, which means
to put the digit'th parenthesized subexpression into the
replacement. E.g.
> sub("([[:alpha:]]+) +([[:alpha:]]+)", "\\2 \\1", "One two three
four five")
[1] "two One three four five"
> gsub("([[:alpha:]]+) +([[:alpha:]]+)", "\\2 \\1", "One two three
four five")
[1] "two One four three five"
To support this without ambiguity or surprises, \\ is expected
to be followed by a digit (or L or U when perl=TRUE).
When fixed=TRUE then there is no possibility of a parenthesized
subexpression so \\2 is taken literally.
help(gsub) is not explicit about this behavior.
Because I initially made the same mistake, when I wrote the S+
versions of gsub and sub I included a warning when the replacement
included a \\ not followed by a digit:
> gsub("([[:alpha:]]+) +([[:alpha:]]+)", "\\ \\", "One two three four
five")
[1] " five"
Warning messages:
backslash in replacement argument of substituteString(fixed=F) is
not
followed by backslash or digit, hence backslash is omitted in:
substit\
uteString(pattern = pattern, replacement = replacement, x = x,
extended ....
> The same issue applies to gsub.
>
> --please do not edit the information below--
>
> Version:
> platform = i386-pc-mingw32
> arch = i386
> os = mingw32
> system = i386, mingw32
> status =
> major = 2
> minor = 8.1
> year = 2008
> month = 12
> day = 22
> svn rev = 47281
> language = R
> version.string = R version 2.8.1 (2008-12-22)
>
> Windows XP (build 2600) Service Pack 2
>
> Locale:
> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
> States.1252;LC_MONETARY=English_United
> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
>
> Search Path:
> .GlobalEnv, package:stats, package:graphics,
> package:grDevices, package:utils, package:datasets,
> package:methods, Autoloads, package:base
>
> --
> Sincerely,
> Andriy
>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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