From: Jeff Wood <jeff.darklight@...>
Date: 2005-08-25T01:09:20+09:00
Subject: Re: Novice Q: What's the difference between /\s*/ and /(\s)*/?

Regarding nil values for groups

if you define your regular expressions like this :
 
a = "My phone number is : 555-1212"
b = /((\d{3})\-)?(\d{3})\-(\d{4})/
c = b.match( a )
puts c.to_a

c should be [ "555-1212", nil, nil, "555", "1212" ]

[0] holds a copy of the complete match
[1] matches the parens from char 0 through char 10
      - The following question mark states that either 0 or 1 instance of the 
      previous group should be accepted in the pattern match.
[2] matches the parens from char 1 through char 7
[3] matches the parens from char 12 through char 18
[4] matches the parens from char 21 through char 27 

I hope that made sense too.

j.


On 8/24/05, David A. Black <dblack@wobblini.net> wrote:
> Hi --
> 
> On Thu, 25 Aug 2005, Jeff Wood wrote:
> 
> > Ok, now for a clean and simple answer...
> >
> > The parenthesis create "groups".  Groups give you the ability to save
> > away parts of the matched pattern for easy access after a match has
> > been made. And yes, the O'Reilly book "Mastering Regular Expressions"
> > would be a good read it explains this concept quite completely.
> >
> > When you use Regexp#match, if you find a match, you are returned a
> > MatchData object which provides an [] API for standard array style
> > access.
> >
> > Element 0 ( [0] ) provides the COMPLETE match. If you defined any
> > groups in the regular expression, they will then appear as additional
> > elements in the MatchData [] array. Each group is then accessable
> > using ordinals ( [1] .. [9] I don't know what happens after 9, I've
> > never needed that many ) ...
> 
> I believe that would be 10 :-)
> 
>    irb(main):003:0> m = /(((((((((((a)))))))))))/.match("a")
>    => #<MatchData:0xbf4bbd84>
>    irb(main):004:0> $10
>    => "a"
>    irb(main):005:0> m[10]
>    => "a"
> 
> 
> > Remember that if you define a group to be
> > inside an optional region of the regular expression, that group will
> > return nil.
> 
> I'm not sure what you mean there.
> 
>    irb(main):009:0> /((a)?)?/.match("a").to_a
>    => ["a", "a", "a"]
> 
> 
> David
> 
> --
> David A. Black
> dblack@wobblini.net
> 
> 


-- 
"So long, and thanks for all the fish"

Jeff Wood