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use strict;
use warnings;
...
  }
  else { print "$3 seems valid to me\n" }
}

while  ( $string =~ /((?:A|B)\d{3})/g ) {
  if ( $1 eq 'A000' or 
...
     print "$1 matched\n";
       }
  else { print "$1 did not match\n" }

/(A(?:000|(?:1(?:2[0-3]|[01]\d))|[23]\d{2}|4(?:[0-4]\d|5[0-6])|999)|B(
+?:000|789|79\d|(?:8[0-7]\d|88[0-8]+)))/