[R] regex -> negate a word

Rau, Roland Rau at demogr.mpg.de
Sun Jan 18 19:35:43 CET 2009


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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