I was inspired to benchmark your advice. In my testing environment, I was astonished to find out that the speed increases as you collect more and more letters (first letters, then second letters, etc.) in your look-ahead. 
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) @ 10
+0.00/s (n=200)
nolookahead:  7 wallclock secs ( 6.30 usr +  0.00 sys =  6.30 CPU) @ 3
+1.75/s (n=200)

[download]
I used the following code and used this document as my input. 
#!/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;
    },
    });

[download]

In reply to Re^2: Fast searching of multiple substrings in a string by betterworld
in thread Fast searching of multiple substrings in a string by dgaramond2

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