Hi, new questions go in Seekers Of Perl Wisdom because

Roy Johnson, whom you asked a question, hasn't been here in 6 weeks.

You used code tags and put your code in between, that is awesome :)

Welcome, see How do I post a question effectively?, Where should I post X?

The regex which is not working for you, contains A zero-width negative look-ahead assertion, and like perlre#(?!pattern) says

A zero-width negative look-ahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar". Note however that look-ahead and look-behind are NOT the same thing. You cannot use this for look-behind.

If you are looking for a "bar" that isn't preceded by a "foo", /(?!foo)bar/ will not do what you want. That's because the (?!foo) is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. Use look-behind instead (see below).

So, use a look-behind

But, that probably won't work either, because you can't have variable length length lookbehind , so you need to use a fixed width lookbehind.

#!/usr/bin/perl --
use strict; use warnings; 
use Test::More qw' no_plan ';
Main(@ARGV);
exit(0);
sub Main {
    my @yesWant = (
        'equity, private equity',
        'equity',
        'private equity,equity',
        'private equity, equity',
        'equity,private equity',
    );
    my @notWant = (
        'private equity',
        'private equity',
        'mutual funds',
        'cds',
    );
    for my $not ( @notWant ){
        ok(  (not TestEquity($not)), "not '$not'" );
    }
    for my $yes ( @yesWant ){
        ok(  TestEquity($yes), "yes '$yes'" );
    }
}
sub TestEquity {
    return 1 if $_[0] =~ m/(?<!private\s)equity/;
    return 0;
}

__END__
$ prove -v pm911357.lookbehind.pl
pm911357.lookbehind.pl ..
ok 1 - not 'private equity'
ok 2 - not 'private equity'
ok 3 - not 'mutual funds'
ok 4 - not 'cds'
ok 5 - yes 'equity, private equity'
ok 6 - yes 'equity'
ok 7 - yes 'private equity,equity'
ok 8 - yes 'private equity, equity'
ok 9 - yes 'equity,private equity'
1..9
ok
All tests successful.
Files=1, Tests=9,  0 wallclock secs ( 0.06 usr +  0.01 sys =  0.08 CPU
+)
Result: PASS
[download]

If fixed width lookbehind doesn't work for you, simply do TWO tests

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