simply regex quantifiers and parse IPs to lowercase

Use ? instead of {0,1} and {3} instead of {3,3} where applicable.
Number groups [ß-9] are left as is for readability, i.e. prefer
[1-9][0-9] over [1-9]\d.

The whole expression is evaluated case-insensitive now, so we can omit
the "a-fA-F" and simply use "a-f".

Unit-tests extended accordingly.

test/StatifyBlacklist_Test.php

@@ -283,6 +283,20 @@ class StatifyBlacklist_Test extends PHPUnit\Framework\TestCase {
 				array( '2001:db8:a0b:12f0::123:456', '2001:db8:a0b:12f0::1/96 ' )
 			)
 		);
+		$this->assertTrue(
+			invoke_static(
+				StatifyBlacklist::class,
+				'cidr_match',
+				array( '2001:DB8:A0B:12F0::123:456', '2001:db8:a0b:12f0::1/96 ' )
+			)
+		);
+		$this->assertTrue(
+			invoke_static(
+				StatifyBlacklist::class,
+				'cidr_match',
+				array( '2001:db8:a0b:12f0::123:456', '2001:DB8:A0B:12F0::1/96 ' )
+			)
+		);
 		$this->assertFalse(
 			invoke_static(
 				StatifyBlacklist::class,