It's fairly easy to create a single high-performance regex that will capture the first (or every) valid float in a string. I would think that the main reason to use two regexes (one to cast a broad net, and one to validate it) would be to helpfully report syntax errors instead of skipping over them and reporting a more generic error. Is that why you're trying to do this?

I'm also not clear on your question, really. (but, I also don't have the book you are referencing)

my $lookAhead = qr/ (?! (?: .*\.){2,}) /x;
my $regex = qr/ ^ $lookAhead [+-]? [\d.]+ $/x;
...
for my $str (@strings) {
    say "\$str => $str";
    if ($str =~ / [+-]?[\d.]+ /x) {  # Pattern fails without this step
+; why???
        if ($& =~ $regex) {
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Your $regex uses '^' and '$', so of course you would need to load the digits into an isolated string first, so I'm guessing I don't understand the question. Could you show an example of the code construct that fails that you think should succeed?