R help - May 2005 - Backslash

Why sometimes one has to put a double
backslash in regular expressions, but
often simple backslashes work too?
Is only a \ required for giving a
metacharacter its usual meaning?
---------------------------------------
u=grep('\\{[\\-u]x',a,perl=T)

       # equivalent to

u=grep('\{[\-u]x',a,perl=T)

       # but

u=grep('\w',a,perl=T)

       # is not correct and requires

u=grep('\\w',a,perl=T)
---------------------------------------
Josef Eschgf??ller

-- 
Josef Eschgf??ller
Dipartimento Matematico
Universita' di Ferrara
http://felix.unife.it

Josef Eschgfaeller wrote:
> Why sometimes one has to put a double
> backslash in regular expressions, but
> often simple backslashes work too?
> Is only a \ required for giving a
> metacharacter its usual meaning?
The general reason is that both R and grep use \ as an escape character. 
  If you want to send an escape to grep you need to escape the escape in R.
> ---------------------------------------
> u=grep('\\{[\\-u]x',a,perl=T)
> 
>        # equivalent to
> 
> u=grep('\{[\-u]x',a,perl=T)
No.  The first one passes the string containing "\{[\-u]x" to grep 
(after processing the escapes).  The second one passes "{[-u]x" to
grep.
> 
>        # but
> 
> u=grep('\w',a,perl=T)
\w has no special meaning in R, so this pattern becomes
"w".
> 
>        # is not correct and requires
> 
> u=grep('\\w',a,perl=T)
This is the pattern "\w".  I don't know if this has special
meaning to
pcre, but I guess it must, or you wouldn't have tried it.

Duncan Murdoch
> ---------------------------------------
> Josef Eschgf??ller
> 
> 
> 
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