[R] Logic with regexps

[(Ted Harding)]

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'?

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.

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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: 10:38:45
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