[R] Logic with regexps

[Prof Brian Ripley]

I think you have missed grepl(), e.g.

X[grepl(rex1, X) & !grepl(rex2, X)]

grepl is a fairly recent addition (2.9.0) that is used extensively in 
R's own text-processing operations (e.g. help files, utilities such as 
'R CMD check').

On Sat, 12 Jun 2010, Ted.Harding at manchester.ac.uk wrote:

> Greetings,
> The following question has come up in an off-list discussion.
> Is it possible to construct a regular expression 'rex' out of
> two given regular expressions 'rex1' and 'rex2', such that a
> character string X matches 'rex' if and only if X matches 'rex1'
> AND X does not match 'rex2'?

Not in general.

> The desired end result can be achieved by logically combining
> the results of a grep using 'rex1' with the results of a grep
> on 'rex2', illustrated by the following example:
>
> ## Given character vector X (below), and two regular exdpressions
> ## rex1="abc", rex2="ijk", to return the elements of X which match
> ## rex1 AND do not match rex1:
> X <- c(
>  "abcdefg",       # Yes
>  "abchijk",       # No
>  "mnopqrs",       # No
>  "ijkpqrs",       # No
>  "abcpqrs" )      # Yes
> rex1 <- "abc"
> rex2 <- "ijk"
> ix1<- grep(rex1,X)
> ix2<- grep(rex2,X)
> X[ix1[!(ix1 %in% ix2)]]
> ## [1] "abcdefg" "abcpqrs"
>
> Question: is there a way to construct 'rex' from 'rex1' and 'rex2'
> such that
>
>  X[grep(rex,X)]
>
> would given the same result?
>
> I've not managed to find anything helpful in desciptions of
> regular expression syntax, though one feels it should be possible
> if this is capable of supporting a logically complete language!
>
> With thanks,
> Ted.

-- 
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