From: Tom Gilbert <tom@...>
Date: 2002-08-10T10:10:52+09:00
Subject: Re: The problem with using $1 in regexps

* Philip Mak (pmak@animeglobe.com) wrote:
> sub _num_quotes {
>     $_[0] =~ /^(>+)/;
>     return length($1);
> }
> 
> See the problem in the above subroutine? (It's Perl, but the $1
> problem applies to Ruby too.) The subroutine is meant to return the
> number of '>' marks at the beginning of a line. However, I
> (mistakenly) wrote a regular expression that will fail to match if
> there are no '>' marks. If it fails to match, I end up returning the
> length of an uninitialized variable. This slipped past my unit tests
> since in the tests, $1 would always start out undefined. The simple
> bug of using /^(>+)/ instead of /^(>*)/ took me a while to track down.
> 
> How would a good programmer avoid this situation? In Ruby, I prefer
> the =~ and $1 regexp syntax (rather than the OO one) because it's shorter
> to write...

Well I'd do:

return length($1) if $_[0] =~ /^(>+)/;
return 0; # or whatever a failed result should be.

I don't know what a "good programmer" would do :)

Tom.
-- 
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   /V\    | Tom Gilbert, London, England | http://linuxbrit.co.uk |
 /(   )\  | Open Source/UNIX consultant  | tom@linuxbrit.co.uk    |
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