if( $sid =~ /^[^0-9]$/ ) {
    # exit here......
}

##</code><code>##

if( $sid !~ m/^([0-9]{1,5})$/ ) {
    # exit
} else {
    $input = $1;
}

##</code><code>##

if( $sid =~ m/^([0-9]{1,5})$/ ) {
    $input = $1;
} else {
    # exit
}