[R] how to replace a single forward slash with a double backward slash in a string?

[David Winsemius]

On Dec 13, 2009, at 1:58 PM, Peter Dalgaard wrote:

> David Winsemius wrote:
>> On Dec 13, 2009, at 1:00 PM, Sean Zhang wrote:
>>> Dear R-helpers.
>>>
>>> Can someone kindly tell me how to replace a single forward slash  
>>> with double
>>> backward slash in a string?
>>>
>>> i.e.,  from  "a/b" to "a\\b"
>> > sub("/", "\\\\","a/b")
>> #the backslashes need to be doubled because they are the escape  
>> character.
>> [1] "a\\b"
>
> Bzzzt. Wrong answer.
>
> > nchar(sub("/", "\\\\","a/b") )
> [1] 3
> > cat(sub("/", "\\\\","a/b"),"\n")
> a\b
>
> > nchar(sub("/", "\\\\\\\\","a/b") )
> [1] 4
> > sub("/", "\\\\\\\\","a/b")
> [1] "a\\\\b"
> > cat(sub("/", "\\\\\\\\","a/b"),"\n")
> a\\b
>
> You are not the first... ;-)

Thanks. I'm probably not the last, but seeing as this is not my first  
effort at this either, let me make an effort toward preventing myself  
(and Zhang) from getting it wrong (again).
-- The console display is a potentially misleading representation of  
the string object in the sense that it displays any "\" characters as  
"\\"?
-- When used in a regular expression, one needs to use FOUR "\"'s to  
get one 'internal' "\", the first two "\\" create one escape-char  
within the parser then which then applies to the second escape-char  
created by the second two "\\"?

(I think I have seen both those facts in print before, but perhaps  
typing them will make it stick this time.)


>
> -- 
>   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
> (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45)  
> 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45)  
> 35327907

David Winsemius, MD
Heritage Laboratories
West Hartford, CT