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.hard...@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, rip...@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 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
