/^(\d+)(??{$^N >=0 && $^N <= 100 ? "" : "(?!)"})$/

##</code><code>##

/^(\d+)(?(?{ $^N < 0 || $^N > 100 })(?!))$/

##</code><code>##

/^(\d+)$(?(?{ $^N < 0 || $^N > 100 })(?!))/