# /a\d+b/
while (<DATA>) {
   local our $incomplete;
   my $match = /
      a
      (?:$(?{$incomplete=1})(?!)|(?(?{$incomplete})(?!))
         \d+
         (?:$(?{$incomplete=1})(?!)|(?(?{$incomplete})(?!))
            b
         )
      )
   /x;

   my $rv = $match      ? "match"
          : $incomplete ? "incomplete"
          :               "no match";

   chomp;
   printf("%-10s  %s\n", $_, $rv);
}

__DATA__
a123b
a
a1
a123
a123c
a123ca123b
a123ca123
a123ca123c