Regexp for validating a UK postcode.

gistfile1.rb

require 'rspec/autorun'

ValidUKPostcode = /\A(
  [A-PR-UWYZ]\d{1,2}                  \s? \d[ABD-HJLNP-UWXYZ]{2}| # A9 9AA or A99 9AA
  [A-PR-UWYZ][A-HK-Y]\d{1,2}          \s? \d[ABD-HJLNP-UWXYZ]{2}| # AA9 9AA or AA99 9AA
  [A-PR-UWYZ]\d[A-HJKSTUW]            \s? \d[ABD-HJLNP-UWXYZ]{2}| # A9A 9AA
  [A-PR-UWYZ][A-HK-Y]\d[ABEHMNPRVWXY] \s? \d[ABD-HJLNP-UWXYZ]{2}| # AA9A 9AA
  GIR                                 \s? 0AA
)\z/ix

describe ValidUKPostcode do
  # http://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom
  [
    'EC1A 1BB',
    'W1A 1HQ',
    'M1 1AA',
    'B33 8TH',
    'CR2 6XH',
    'DN55 1PT'
  ].each do |valid_postcode|
    it { should match(valid_postcode) }
  end

  [
    '123 456',
    'ABC DEF',
    'M1 1AA1'
  ].each do |invalid_postcode|
    it { should_not match(invalid_postcode) }
  end
end