From: Kevin Olbrich <devlists-ruby-talk@...>
Date: 2006-03-29T00:40:01+09:00
Subject: Re: Regexp gotcha

You could just do this...

if string =~ /(\w)/
  #do something with $1
end

anytime you try to match a string to a non-regexp object you get a  
false, I think.

_Kevin


On Wednesday, March 29, 2006, at 12:24 AM, Pistos Christou wrote:
>Hi, all.  I was fixing a bug last night, and discovered some
>"gotcha"-like behaviour in the process.  Consider:
>
>irb(main):173:0> s = "my string"
>=> "my string"
>irb(main):174:0> r1 = /my/
>=> /my/
>irb(main):175:0> r2 = /your/
>=> /your/
>irb(main):176:0> r3 = nil
>=> nil
>irb(main):177:0> s =~ r1
>=> 0
>irb(main):178:0> s =~ r2
>=> nil
>irb(main):179:0> s =~ r3
>=> false
>
>s =~ r1  .... That's cool, it gives me the index of the match.
>s =~ r2  .... That's cool, it tells me there was no match.
>s =~ r3  .... Whoa.
>
>The reason this "got me" is that I had this code:
>
>match_result = ( some_string =~ some_regexp )
>if match_result != nil
>    # Assume there was a match
>end
>
>But the problem is... I had an s =~ r3 case because some_regexp was nil,
>and so it was entering my if block when I semantically did not want that
>to occur.  :(
>
>So now my code is
>
>if match_result != nil and match_result != false
>...
>end
>
>Note also that I can't even use Regexp.last_match != nil in my if block:
>
>irb(main):224:0> s =~ r2
>=> nil
>irb(main):225:0> Regexp.last_match
>=> nil
>irb(main):226:0> s =~ r3
>=> false
>irb(main):227:0> Regexp.last_match
>=> nil
>irb(main):228:0> s =~ r1
>=> 0
>irb(main):229:0> Regexp.last_match
>=> #<MatchData:0x406c40b4>
>irb(main):230:0> s =~ r3
>=> false
>irb(main):231:0> Regexp.last_match
>=> #<MatchData:0x406c40b4>
>
>To be clear: Note how a "nil type" of non-match overwrites last_match,
>but a "false type" of non-match doesn't.
>
>So the question is... why are BOTH nil and false possible return values
>of =~ ?  Is there some benefit to this?  Why not just one or the other?
>
>I see that this behaviour is [documented][1] but I still feel that this
>is unintuitive behaviour when people assume =~ only applies to Regexp
>RHS's.
>
>[1]: http://www.ruby-doc.org/core/classes/String.html#M001453
>
>Thanks in advance for any and all clarifications and explanations.
>
>Pistos
>
>--
>Posted via http://www.ruby-forum.com/.
>





-- 
Posted with http://DevLists.com.  Sign up and save your time!