#!/usr/local/bin/perl -w
#
use strict;

# Make some building blocks. What we want at the start; what we don't want
# after "foobar:" using a negative look-ahead assertion; any five
# characters; what we want at the end (which we will discard later).
#
our $rxFooBar = qr{foobar:};
our $rxNotGonkish = qr{(?!(?:g(?:o(?:n(?:k)*)*)*|@))};
our $rxFive = qr{.{1,5}};
our $rxGonk = qr{(?:gonk|@)};

# Put the building blocks together using "()" to capture everything except
# the "gonk" or "@" at the end;
#
our $rxPutItAllTogether = qr{($rxFooBar$rxNotGonkish$rxFive)$rxGonk};

while(<DATA>)
{
    chomp;
    print "$_\n";
    print /$rxPutItAllTogether/ ? "$1\n\n" : "Failed\n\n";
}

__END__
foobar:hellogonk
foobar:gonk
foobar:higonk
foobar:helloworldgonk
foobar:ggonk
foobar:gogonk
foobar:gongonk
foobar:gonkgonk
foobar:gonkygonk
foobar:@gonk
foobar:its@
foobar:toomany@