The proper way to protect yourself from using unintended old values in $1 and friends is to program defensively and check if a pattern match succeeded before trying to use captured subexpressions (as previous messages in this thread have shown).
Well, that's a way. Another way is to stylistically outlaw all uses of $1 et seq, except in the right side of a substitution. Any other "capturing" should be done as list-context assignment: 
my ($first, $second) = $source =~ /blah(this)blah(that)blah/;

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Then it's very clear what the scope and origination of $first and $second are.

-- Randal L. Schwartz, Perl hacker

In reply to Re: Re: Re: Regular Expression Question by merlyn
in thread What happens with empty $1 in regular expressions? (was: Regular Expression Question) by marcblecher

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