My actual data file has thousands of lines of text and much more text on each line than a single word, but still, not using the negative look ahead runs against these large files in less than a second. Why would this cause such a slow down? Is there a way around this?

If I am reading hipowls's regex correctly it will be checking the negative look-ahead for every character between 'start' and 'end'. Just doing the look-ahead once should locate the last 'start' in a group then the .+? can run without keep checking after every character.

use strict;
use warnings;

my $string = <<'EOT';
start
start
start
go
one
end
start
start
start
go
two
end
EOT

my $rxGroup = qr
   {(?isx)
       (
           start
           (?!\nstart)
           .+?
           end
       )
   };

print qq{$1\n\n} while $string =~ m{$rxGroup}g;
[download]

The output.

start
go
one
end

start
go
two
end
[download]

I hope I am correct and this slight change will speed up your code.

Cheers,

JohnGG

That assumes that the starts are on consecutive lines which may be a perfectly valid assumption. It pays to know your data.

Another approach is to use the original regex, which may have multiple starts and then trim it using s/^.*start/start/is.

The loop then looks something like

while ( $string =~ /(start.+?end)/gis ) {
    my $data = $1;
    $data =~ s/^.*start/start/is;
    print $data, "\n\n";
}
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If the intent is to strip off multiple starts only on consecutive lines then the regex would be s/^(?:start\s*)+start/start/is which used on the input

start
start
start
go
one
end
start
start
data
start
go
two
end
[download]

would produce

start
go
one
end

start
data
start
go
two
end
[download]

But as I said you really need to know your data and other factors such as if you need to validate the input.