From: Philip Mak <pmak@...>
Date: 2002-08-10T10:07:07+09:00
Subject: The problem with using $1 in regexps

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...