Update UUID pattern to accept the 'nil' UUID.

Fixes #4133

lib/Cake/Test/Case/Utility/ValidationTest.php

@@ -1924,6 +1924,7 @@ class ValidationTest extends CakeTestCase {
 	}
 
 	public function testUuid() {
+		$this->assertTrue(Validation::uuid('00000000-0000-0000-0000-000000000000'));
 		$this->assertTrue(Validation::uuid('550e8400-e29b-11d4-a716-446655440000'));
 		$this->assertFalse(Validation::uuid('BRAP-e29b-11d4-a716-446655440000'));
 		$this->assertTrue(Validation::uuid('550E8400-e29b-11D4-A716-446655440000'));

lib/Cake/Utility/Validation.php

@@ -797,7 +797,7 @@ class Validation {
  * @return boolean Success
  */
 	public static function uuid($check) {
-		$regex = '/^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[1-5][a-fA-F0-9]{3}-[89aAbB][a-fA-F0-9]{3}-[a-fA-F0-9]{12}$/';
+		$regex = '/^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[0-5][a-fA-F0-9]{3}-[089aAbB][a-fA-F0-9]{3}-[a-fA-F0-9]{12}$/';
 		return self::_check($check, $regex);
 	}