#!/usr/bin/perl -l

$zipadidoda = 'le12 ogx';

print "In: $zipadidoda";
if( $zipadidoda =~ s/^([A-Z]{1,2}\d{1,2}[A-Z]?)\s?(O|\d)([A-Z]{2})$/uc
+("$1 ".(0+$2).$3)/ie ) {
        print "Out: $zipadidoda";
} else {
        print "Invalid zipcode";
}
[download]

You wanted a single re, so you got one!

Or is the 'e' modified considered cheating?

:)

Update: removed redundant quotes

Update: and fixed the space after a tip from ww