Benchmark: timing 200 iterations of lookahead1, lookahead3, lookahead4, lookahead5, nolookahead...
lookahead1:  3 wallclock secs ( 2.93 usr +  0.00 sys =  2.93 CPU) @ 68.26/s (n=200)
lookahead3:  2 wallclock secs ( 2.23 usr +  0.00 sys =  2.23 CPU) @ 89.69/s (n=200)
lookahead4:  2 wallclock secs ( 2.03 usr +  0.00 sys =  2.03 CPU) @ 98.52/s (n=200)
lookahead5:  2 wallclock secs ( 2.00 usr +  0.00 sys =  2.00 CPU) @ 100.00/s (n=200)
nolookahead:  7 wallclock secs ( 6.30 usr +  0.00 sys =  6.30 CPU) @ 31.75/s (n=200)
##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark;

undef $/;
my $text = <>;

# "progressive", "great", "interlacing", "really" and "wonder" are in the document
my $words = q{
    featuring |
    blossom |
    great |
    interlacing |
    really |
    linux |
    thought |
    wonder |
    progressive
};

timethese(200, {
	lookahead5 => sub {
	    1 while $text =~ m{(?=
		[fbgrltwpi][elihorn][aoent][tsaludge][ustrlxger]
	    )(
	        $words
	    )}gix;
	},
	lookahead4 => sub {
	    1 while $text =~ m{(?=
		[fbgrltwpi][elihorn][aoent][tsaludge]
	    )(
	        $words
	    )}gix;
	},
	lookahead3 => sub {
	    1 while $text =~ m{(?=
		[fbgrltwpi][elihorn][aoent]
	    )(
	        $words
	    )}gix;
	},
	lookahead1 => sub {
	    1 while $text =~ m{(?=
		[fbgrltwpi]
	    )(
	        $words
	    )}gix;
	},
	nolookahead => sub {
	    1 while $text =~ m{(
		$words
	    )}gix;
	},
    });