[R] Logic with regexps
Prof Brian Ripley
ripley at stats.ox.ac.uk
Sat Jun 12 14:46:57 CEST 2010
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:
> 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)]]
> ##  "abcdefg" "abcpqrs"
> Question: is there a way to construct 'rex' from 'rex1' and 'rex2'
> such that
> 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,
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
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