#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11132725
use warnings;

while( <DATA> )
  {
  print /(?<!\d)5\d{8}(?!\d)/ ? '   ' : 'not', " found in : ", $_;
  }

__DATA__
for me. "503009496" and i w
for me. "603055496" and i w (6)
for me. "703509496" and i w (7)
for me. "53009496" and i w (too short)
for me. "5093009496" and i w (too long)
for me. "503009496" and i w