($\, $,) = ("\n", "\t");
print $_, /\b(\d+)\b(?(?{$1 >= 256})(?!))/ ? 'Y' : 'N' for 0 .. 1000;

##</code><code>##

    /\b(\d+)\b/

##</code><code>##

    (?(condition)yes-pattern)

##</code><code>##

(?{ code })

##</code><code>##

    (?!)

##</code><code>##

/^(\d+)(?(?{$1 >= 256})(?!))\.(\d+)(?(?{$2 >= 256})(?!))\.(\d+)(?(?{$3 >= 256})(?!))\.(\d+)(?(?{$4 >= 256})(?!))$/

##</code><code>##

/^(\d+)\.(\d+)\.(\d+)\.(\d+)$(?(?{$1 >= 256 || $2 > 256 || $3 >= 256 ||$4 >= 256})(?!))/