Thank you very much to all of you for your fast and excellent help.
Since the "-grep(...)" solution seems to be favored by most of the answers, I just wonder if there is really no regular expression which does the job?!?

Thanks again,
Roland



-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck@gmail.com]
Sent: Sun 1/18/2009 8:28 PM
To: Rau, Roland
Cc: r-help@r-project.org
Subject: Re: [R] regex -> negate a word
 
Try this:

# indexes
setdiff(seq_along(x), grep("abc", x))

# values
setdiff(x, grep("abc", x, value = TRUE))

Another possibility is:

z <- "abc"
x0 <- c(x, z) # to handle no match case
x0[- grep(z, x0)] # values




On Sun, Jan 18, 2009 at 1:35 PM, Rau, Roland <Rau@demogr.mpg.de> wrote:
> Dear all,
>
> let's assume I have a vector of character strings:
>
> x <- c("abcdef", "defabc", "qwerty")
>
> What I would like to find is the following: all elements where the word
> 'abc' does not appear (i.e. 3 in this case of 'x').
>
> Since I am not really experienced with regular expressions, I started
> slowly and thought I find all word were 'abc' actually does appear:
>
>> grep(pattern="abc", x=x)
> [1] 1 2
>
> So far, so good. Now I read that ^ is the negation operator. But it can
> also denote the beginning of a string as in:
>
>> grep(pattern="^abc", x=x)
> [1] 1
>
> Of course, we need to put it inside square brackets to negate the
> expression [1]
>> grep(pattern="[^abc]", x=x)
> [1] 1 2 3
>
> But this is not what I want either.
>
> I'd appreciate any help. I assume this is rather easy and
> straightforward.
>
> Thanks,
> Roland
>
>
> [1] http://www.zytrax.com/tech/web/regex.htm: The ^ (circumflex or
> caret) inside square brackets negates the expression....
>
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