This is the regex I use for checking UK postcodes. It is similar, but uses alternation instead...

# Check UK postcode format
# Format is LD DLL, LLD DLL, LDD DLL, or LLDD DLL where L = letter, D 
+= digit
$postcode = uc($postcode); # Could be removed if you use the /i regex 
+switch
if ($postcode !~ /^(([A-Z]\d|[A-Z][A-Z]\d|[A-Z]\d\d|[A-Z][A-Z]\d\d)\s(
+\d|o)[A-Z][A-Z])$/i) {
    $error_msg .= "Invalid postcode for UK.<br /><br />\n";
}
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The (\d|o) has the letter o to cater for an incorrect o instead of a zero. To actually change it to a zero would require a second line (using the substitution operator). I don't know of any way to make that change in the regex line while it is being parsed... UPDATE: Looks like there are a couple of ways to do it in one line, including changing an incorrect letter o to a zero. I stand corrected :)