From: dblack@...
Date: 2006-03-29T00:33:24+09:00
Subject: Re: Regexp gotcha

Hi --

On Wed, 29 Mar 2006, 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

You could shorten that to:

   if match_result

I'm not sure about the "why" part of the nil/false thing.


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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