public static function is_gb_postcode( $to_check ) {

		// Permitted letters depend upon their position in the postcode.
		// https://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom#Validation.
		$alpha1 = '[abcdefghijklmnoprstuwyz]'; // Character 1.
		$alpha2 = '[abcdefghklmnopqrstuvwxy]'; // Character 2.
		$alpha3 = '[abcdefghjkpstuw]';         // Character 3 == ABCDEFGHJKPSTUW.
		$alpha4 = '[abehmnprvwxy]';            // Character 4 == ABEHMNPRVWXY.
		$alpha5 = '[abdefghjlnpqrstuwxyz]';    // Character 5 != CIKMOV.

		$pcexp = array();

		// Expression for postcodes: AN NAA, ANN NAA, AAN NAA, and AANN NAA.
		$pcexp[0] = '/^(' . $alpha1 . '{1}' . $alpha2 . '{0,1}[0-9]{1,2})([0-9]{1}' . $alpha5 . '{2})$/';

		// Expression for postcodes: ANA NAA.
		$pcexp[1] = '/^(' . $alpha1 . '{1}[0-9]{1}' . $alpha3 . '{1})([0-9]{1}' . $alpha5 . '{2})$/';

		// Expression for postcodes: AANA NAA.
		$pcexp[2] = '/^(' . $alpha1 . '{1}' . $alpha2 . '[0-9]{1}' . $alpha4 . ')([0-9]{1}' . $alpha5 . '{2})$/';

		// Exception for the special postcode GIR 0AA.
		$pcexp[3] = '/^(gir)(0aa)$/';

		// Standard BFPO numbers.
		$pcexp[4] = '/^(bfpo)([0-9]{1,4})$/';

		// c/o BFPO numbers.
		$pcexp[5] = '/^(bfpo)(c\/o[0-9]{1,3})$/';

		// Load up the string to check, converting into lowercase and removing spaces.
		$postcode = strtolower( $to_check );
		$postcode = str_replace( ' ', '', $postcode );

		// Assume we are not going to find a valid postcode.
		$valid = false;

		// Check the string against the six types of postcodes.
		foreach ( $pcexp as $regexp ) {
			if ( preg_match( $regexp, $postcode, $matches ) ) {
				// Remember that we have found that the code is valid and break from loop.
				$valid = true;
				break;
			}
		}

		return $valid;
	}