I would add \b boundaries at the front and rear else for example, 2/365555555 is a valid number. m{\b$number x $number x $number/$number\b} Update: alternatively using {\A$number x $number x $number/$number\z} or {^$number x $number x $number/$number$} also looks fine to me.

In addition, for even more potential validation of this string, the number itself could be made capturing or put parens in the longer regex.

if ( my($n1,$n2,$n3,$n4) = string =~ {\A($number) x ($number) x ($numb
+er)/($number)\z} and
        $n1 == $n2 )
{ valid format and square}
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