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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