From: Andrew Johnson <ajohnson@...>
Date: 2005-01-27T16:10:59+09:00
Subject: Re: regex questions

On Thu, 27 Jan 2005 12:21:35 +0900, Jeff Davis <jdavis-list@empires.org> wrote:
> In python the regexes allow you to call a function instead of just 
> substitute the values (see <http://docs.python.org/lib/node111.html> for 
> more details). That seems quite useful, is there something similar in ruby?

Yes, #sub and #gsub can be passed a block to be evaluated for
the replacement value.
 
> Also, let's say I want match anything between "a" and "b" unless it 
> contains the word "foo". I could write two regexes like so:
> 
> if str =~ /a(.*)b/ and str !~ /a(.*foo.*)b/

Careful, consider this string:

  str = 'afoolbalmnbaopbafoob'

which has two sets of a..b that don't contain 'foo', but your
pairing rejects it because a..foo..b can also be found.


> Is there a good way to make that kind of logic into one regex? Is there 
> some kind of "intersect" operator or a "not" operator?

A fairly standard way is to use negative look-ahead and inch
ahead one character at a time like:

  re = %r{a((?:(?!foo).)*?)b}
  str = 'afoolbalmnbaopbafoob'
  str.scan(re).each do |m|
    p m
  end
  __END__
  ["lmn"]
  ["op"]

regards,
andrew

-- 
Andrew L. Johnson   http://www.siaris.net/
      But puzzles in programming are what make it challenging and fun
      sometimes... you always end up learning one more way not to do
      something each time.  -- Brad Fenwick