Perl regex with a negative lookahead behaves unexpectedly - regex

Question

I'm attempting to match /ezmlm-(any word except 'weed' or 'return')\s+/ with a regex. The following demonstrates a foreach loop which does the right thing, and an attempted regex which almost does:
#!/usr/bin/perl
use strict;
use warnings;
my #tests = (
{ msg => "want 'yes', string has ezmlm, but not weed or return",
str => q[|/usr/local/bin/ezmlm-reject '<snip>'],
},
{ msg => "want 'yes', array has ezmlm, but not weed or return",
str => [ <DATA> ],
},
{ msg => "want 'no' , has ezmlm-weed",
str => q[|/usr/local/bin/ezmlm-weed '<snip>'],
},
{ msg => "want 'no' , doesn't have ezmlm-anything",
str => q[|/usr/local/bin/else '<snip>'],
},
{ msg => "want 'no' , ezmlm email pattern",
str => q[crazy/but/legal/ezmlm-wacky#example.org],
},
);
print "foreach regex\n";
foreach ( #tests ) {
print doit_fe( ref $_->{str} ? #{$_->{str}} : $_->{str} ) ? "yes" : "no";
print "\t";
print doit_re( ref $_->{str} ? #{$_->{str}} : $_->{str} ) ? "yes" : "no";
print "\t<--- $_->{msg}\n";
};
# for both of the following subs:
# #_ will contain one or more lines of data
# match the pattern /ezmlm-(any word except 'weed' or 'return')\s+/
sub doit_fe {
my $has_ezmlm = 0;
foreach ( #_ ) {
next if $_ !~ m/ezmlm-(.*?)\s/;
return 0 if $1 eq 'weed' or $1 eq 'return';
$has_ezmlm++;
};
return $has_ezmlm;
};
sub doit_re { return grep /ezmlm-(?!weed|return)/, #_; };
__DATA__
|/usr/local/bin/ezmlm-reject '<snip>'
|/usr/local/bin/ezmlm-issubn '<snip>'
|/usr/local/bin/ezmlm-send '<snip>'
|/usr/local/bin/ezmlm-archive '<snip>'
|/usr/local/bin/ezmlm-warn '<snip>'
The output of the sample program is as follows:
foreach regex
yes yes <--- want 'yes', string has ezmlm, but not weed or return
yes yes <--- want 'yes', array has ezmlm, but not weed or return
no no <--- want 'no' , has ezmlm-weed
no no <--- want 'no' , doesn't have ezmlm-anything
no yes <--- want 'no' , ezmlm email pattern
In the last instance, the regex fails, matching a goofy but legal email address. If I amend the regex placing a \s after the negative lookahead pattern like so:
grep /ezmlm-(?!weed|return)\s+/
The regex fails to match at all. I'm supposing it has to do with the how the negative pattern works. I've tried making the negation non-greedy, but it seems there's some lesson buried in 'perldoc perlre' that is escaping me. Is it possible to do this with a single regex?

Answer 1

The negative look-ahead is zero-width which means that the regex
/ezmlm-(?!weed|return)\s+/
will only match if one or more space characters immediately follow "ezmlm-".
The pattern
/ezmlm-(?!weed|return)/
will match
"crazy/but/legal/ezmlm-wacky#example.org"
because it contains "ezmlm-" not followed by "weedy" or "return".
Try
/ezmlm-(?!weed|return)\S+\s+/
where \S+ is one or more non-space characters (or instead use [^#\s]+ if you want to deny email addresses even if followed by a space).