[R] Logic with regexps

[(Ted Harding)]

Thanks, Brian. I had indeed overlooked grepl() (too busy delving
into the syntax summary)! That is certainly a useful shortcut of
the construction I had used.

Your "Not in general" implies that using grep() twice (in this example;
more times in more complex combinations) is inevitable -- which of
course was part of the point of the query!

A very helpful reply. Thanks!
Ted.

On 12-Jun-10 12:46:57, Prof Brian Ripley wrote:
> 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
> 
> ______________________________________________
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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 12-Jun-10                                       Time: 14:14:05
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