Regular Expressions

Hi,

I'm trying to figure out how to use capturing parenthesis in regular 
expressions in R.  (Doing this in Perl, Java, etc. is fairly trivial, but I 
can't seem to find the functionality in R.)

For example, given the string:    "10 Nov 13.00 (PFE1020K13)"

I want to capture the first to digits and then the month abreviation.

In perl, this would be

/^(\d\d)\s(\w\w\w)\s/

Then I have the variables $1 and $1 assigned to the capturing parenthesis.

I've found the grep and sub commands in R, but the docs don't indicate any 
way to capture things.

Any suggestions?

Read the the link to ?regexp.  It *does* 'indicate the way to capture 
things'.

      The backreference ?\N?, where ?N = 1 ... 9?, matches the substring
      previously matched by the Nth parenthesized subexpression of the
      regular expression.  (This is an extension for extended regular
      expressions: POSIX defines them only for basic ones.)

and there is an example on the help page for grep():

      ## Double all 'a' or 'b's;  "\" must be escaped, i.e., 'doubled'
      gsub("([ab])", "\\1_\\1_", "abc and ABC")

In your example

x <- "10 Nov 13.00 (PFE1020K13)"
regex <- "(\\d\\d)\\s(\\w\\w\\w).*"
sub(regex, "\\1", x, perl = TRUE)
sub(regex, "\\2", x, perl = TRUE)

A better way to do this would be something like

regex <- "([[:digit:]]{2})\\s([[:alpha:]]{3}).*"

which is also a POSIX extended regexp.

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