From: Robert Klemme <shortcutter@...>
Date: 2010-07-27T05:40:15+09:00
Subject: Re: Regex ^ beginning not strong?

On 07/26/2010 06:01 PM, Iain Barnett wrote:
> Hi,
>
> I've some more regex questions. I wrote a pattern to check for valid
> regexes and inspect the parts (we all have our reasons for the things
> we do:) It wasn't working so I went down to simpler and simpler
> patterns, but I'm a bit surprised at the way Ruby 1.9 is handling the
> regexes. I tested the same pattern in Perl and it came out with the
> answers I'd expect.
>
> Is this down to me using perl regexes for so long, or is there
> something I'm missing about Ruby's implementation? It appears ^ at
> the beginning of a string doesn't bind as strongly as I'd expect.
>
>
> I believe this test should fail as<delim>  should be bound to the
> beginning of the string by the ^ , and the match result is a little
> bit crazy - shouldn't the main capture be "d\\d" if it's following
> the logical route it's chosen? $ ruby -e ' md =
> /^(?<mors>m)?(?<delim>.)(?<pat>.+?)\g<delim>/.match( %q!/\d\d\\d! )
> puts md.inspect ' #<MatchData "/\\d" mors:nil delim:"d" pat:"\\">
>

I think you found a bug - probably related to referring to back 
references to named capturing groups:

irb(main):013:0> s = %q!/\d\d\\d!
=> "/\\d\\d\\d"

irb(main):027:0> r = /^(?<mors>m)?(?<delim>.)(?<pat>.+?)/
=> /^(?<mors>m)?(?<delim>.)(?<pat>.+?)/
irb(main):028:0> md = r.match s
=> #<MatchData "/\\" mors:nil delim:"/" pat:"\\">

This must not match at all:

irb(main):029:0> r = /^(?<mors>m)?(?<delim>.)(?<pat>.+?)\g<delim>/
=> /^(?<mors>m)?(?<delim>.)(?<pat>.+?)\g<delim>/
irb(main):030:0> md = r.match s
=> #<MatchData "/\\d" mors:nil delim:"d" pat:"\\">

It seems to work better with numbered capturing groups

irb(main):027:0> r = /^(?<mors>m)?(?<delim>.)(?<pat>.+?)/
=> /^(?<mors>m)?(?<delim>.)(?<pat>.+?)/
irb(main):028:0> md = r.match s
=> #<MatchData "/\\" mors:nil delim:"/" pat:"\\">
irb(main):029:0> r = /^(?<mors>m)?(?<delim>.)(?<pat>.+?)\g<delim>/
=> /^(?<mors>m)?(?<delim>.)(?<pat>.+?)\g<delim>/
irb(main):030:0> md = r.match s
=> #<MatchData "/\\d" mors:nil delim:"d" pat:"\\">

Normal greediness:

irb(main):035:0> r = /^(m)?(.)(.+)\2/
=> /^(m)?(.)(.+)\2/
irb(main):036:0> md = r.match s
=> nil

This works:

irb(main):038:0> /^(m)?(.)(.+)\2/.match 'abbba'
=> #<MatchData "abbba" 1:nil 2:"a" 3:"bbb">

Maybe the numbering gets out of order if we try to mix:

irb(main):039:0> /^(?<delim>m)?(.)(.+)\2/.match 'abbba'
SyntaxError: (irb):39: numbered backref/call is not allowed. (use name): 
/^(?<delim>m)?(.)(.+)\2/
	from /usr/local/bin/irb19:12:in `<main>'
irb(main):040:0> /^(?<delim>m)?(.)(.+)\k<2>/.match 'abbba'
SyntaxError: (irb):40: numbered backref/call is not allowed. (use name): 
/^(?<delim>m)?(.)(.+)\k<2>/
	from /usr/local/bin/irb19:12:in `<main>'
irb(main):041:0>

irb(main):047:0> RUBY_VERSION
=> "1.9.1"
irb(main):048:0> RUBY_PATCHLEVEL
=> 376

Frankly, I never used named capturing groups yet (simply for habit and 
compatibility).  It was probably a good choice so far.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/