is it possible to put a character group (and quantify it with a * + ? or {} ) into a look-ahead match?

Sure it is. Anything you can do in a plain match, you can do in a lookahead match.

For lookbehind, it's a different matter: you can only use fixed length lookbehind, so quantifiers (like * and +), and varied-length alternatives, are out. BTW if all alternatives have the same length, it is allowed, as in:

$_ = q[There's food at the bar.];
while(/(?<=foo|bar)(\S+)/g) {
    print "$1\n";
}
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