From: Robert Klemme <shortcutter@...>
Date: 2008-05-05T04:10:01+09:00
Subject: Re: Backreferences in case statements

On 04.05.2008 15:21, loveajax@gmail.com wrote:
> On May 4, 3:11 am, Robert Klemme <shortcut...@googlemail.com> wrote:
>> On 03.05.2008 18:00, lovea...@gmail.com wrote:
>>
>>> I am not sure if this is possible. I have a case statement which
>>> checks for a regular expression. For example:
>>> case text
>>> when /(abc)(.)*/
>>> when /(xyz)(.)*/
>>> end
>>> In the above snippet how can I use the instance variables pre_match
>>> and post_match? I tried calling these methods without any qualifiers
>>> but I got a NoMethodError. Where does Ruby store the MatchData if the
>>> matching is done in a case statement?
>> $ irb
>> irb(main):001:0> /b/ =~ "abc"
>> => 1
>> irb(main):002:0> $`
>> => "a"
>> irb(main):003:0> $'
>> => "c"
>> irb(main):004:0> $~
>> => #<MatchData:0x7ff9eb54>
>> irb(main):005:0> $~.to_a
>> => ["b"]
>> irb(main):006:0> $~.pre_match
>> => "a"
>> irb(main):007:0> $~.post_match
>> => "c"
>> irb(main):008:0>
>>
>> But why do you have groups in your regular expressions if you are not
>> interested in the content?  Also "(.)*" seems a bit odd because it might
>> not yield what you expect:
>>
>> irb(main):008:0> /a(.)*/ =~ "abcdef"
>> => 0
>> irb(main):009:0> $1
>> => "f"
>>
>> IMHO it is generally a bad idea to use grouping in the way you do it
>> because it will capture a lot that you are not interested in.  It seems
>> you might rather want "(.*)".

> In my guess /b/ =~ "abc" is what sets $~ variable. Since I am not
> using that statement anywhere that variable is not set at all.

That's complete nonsense.  Did you try it out?  I guess not.  Come on, 
don't be so lazy.

> Instead
> I am doing the matching in a case statement. As a result I am not able
> to do something like $~.pre_match. I am not fond of $1, $2, etc
> variables. I feel comfortable using the instance variables pre_match
> and post_match.

pre_match and post_match are not instance variables but methods of class 
MatchData.

> But you solved the first problem I was trying to solve :) I was using
> (.)* instead of (.*) and I couldn't get what I wanted. (.*) is what I
> needed. Thanks so much for that. Now I can use $1, $2 at least if not
> pre_match and post_match.

irb(main):007:0> /b/ === "abc"
=> true
irb(main):008:0> $~
=> #<MatchData:0x100230b8>
irb(main):009:0> $~.pre_match
=> "a"
irb(main):010:0> $~.post_match
=> "c"
irb(main):011:0>

	robert