[R] grep and gsub on backslash and quotes

[Simon Fear]

Thank you. Single backslash version, first thing I tried (I thought)
works
just fine when I copy and paste, ergo I must have got confused by some
stupid
typo of mine. Sorry to waste everyone's  time over this. (Still, I am
probably not the only confused user when it comes to RE handling - I
hope the
examples posted will be of as much use to others as they are to me.)
Simon


> -----Original Message-----
> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
> Sent: 12 August 2003 17:13
> To: Simon Fear
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] grep and gsub on backslash and quotes
> 
> 
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> ________________________________________________________________
> 
> On Tue, 12 Aug 2003, Simon Fear wrote:
> 
> > The following code works,  to gsub single quotes to double quotes:
> > 
> > line <- gsub("'", '"', line)
> > 
> > (that's a single quote within doubles then a double within 
> singles if
> > your
> > viewer's font is not good).
> > 
> > But The R Language Manual tells me that
> > 
> > Quotes and other special characters within strings
> > are specified using escape sequences:
> > \' single quote
> > \" double quote
> > 
> > so why is the following wrong: gsub("\\\\'", "\\\\"", line)? That or
> any
> > other number of backslashes (have tried all up to n=6 just for good
> > measure).
> > 
> > BTW is it documented anywhere that you need four 
> backslashes in an RE
> to
> > match one in the target, when it is being passed as an argument to
> gsub
> > or
> > grep?
> 
> It's not true, so I hope it is not documented anywhere.  You 
> may need 6,
> as in the following from methods():
> 
>     res <- sort(grep(gsub("([.[])", "\\\\\\1", name), an, 
> value = TRUE))
> 
> since that is \\ \1 withou tthe space.  Each backslash in the target
> only
> needs to be doubled.
> 
> In your example
> 
> gsub("\'", "\"", line) or even gsub("'", "\"", line)
> 
> is all you need: only R strings need the escape.
> 
> 
> -- 
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272866 (PA)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>
 

Simon Fear
Senior Statistician
Syne qua non Ltd
Tel: +44 (0) 1379 644449
Fax: +44 (0) 1379 644445
email: Simon.Fear at synequanon.com
web: http://www.synequanon.com
 
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