Two transformations works well: 
  s/(\w{3,5})\s*(\w{3})/\U$1 $2/;
  s/ O/ 0/;

[download]
Update: conforms to specs cited by Mutant.
Caution: Contents may have been coded under pressure.

In reply to Re: UK postcode regex by Roy Johnson
in thread UK postcode regex by Anonymous Monk

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