Fix #4415 and add unit tests for https://bugs.php.net/bug.php?id=51192
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 3163276..ea3bc6d 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1214,6 +1214,14 @@
 			$str = $matches[2];
 		}
 
+		// PHP 7 accepts IPv6 addresses within square brackets as hostnames,
+		// but it appears that the PR that came in with https://bugs.php.net/bug.php?id=68039
+		// was never merged into a PHP 5 branch ... https://3v4l.org/8PsSN
+		if (preg_match('/^\[([^\]]+)\]/', $str, $matches) && ! is_php('7') && filter_var($matches[1], FILTER_VALIDATE_IP, FILTER_FLAG_IPV6) !== FALSE)
+		{
+			$str = 'ipv6.host'.substr($str, strlen($matches[1]) + 2);
+		}
+
 		$str = 'http://'.$str;
 
 		// There's a bug affecting PHP 5.2.13, 5.3.2 that considers the
diff --git a/tests/codeigniter/libraries/Form_validation_test.php b/tests/codeigniter/libraries/Form_validation_test.php
index 65a3bbf..f455b91 100644
--- a/tests/codeigniter/libraries/Form_validation_test.php
+++ b/tests/codeigniter/libraries/Form_validation_test.php
@@ -231,6 +231,13 @@
 		$this->assertTrue($this->form_validation->valid_url('www.codeigniter.com'));
 		$this->assertTrue($this->form_validation->valid_url('http://codeigniter.com'));
 
+		// https://bugs.php.net/bug.php?id=51192
+		$this->assertTrue($this->form_validation->valid_url('http://accept-dashes.tld'));
+		$this->assertFalse($this->form_validation->valid_url('http://reject_underscores.tld'));
+
+		// https://github.com/bcit-ci/CodeIgniter/issues/4415
+		$this->assertTrue($this->form_validation->valid_url('http://[::1]/ipv6'));
+
 		$this->assertFalse($this->form_validation->valid_url('htt://www.codeIgniter.com'));
 		$this->assertFalse($this->form_validation->valid_url(''));
 		$this->assertFalse($this->form_validation->valid_url('code igniter'));
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index a3fed6a..a8cb3f5 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -17,6 +17,7 @@
 -  Fixed a bug (#4401) - :doc:`Query Builder <database/query_builder>` breaks ``WHERE`` and ``HAVING`` conditions that use ``IN()`` with strings containing a closing parenthesis.
 -  Fixed a regression in :doc:`Form Helper <helpers/form_helper>` functions :php:func:`set_checkbox()`, :php:func:`set_radio()` where "checked" inputs aren't recognized after a form submit.
 -  Fixed a bug (#4407) - :doc:`Text Helper <helpers/text_helper>` function :php:func:`word_censor()` doesn't work under PHP 7 if there's no custom replacement provided.
+-  Fixed a bug (#4415) - :doc:`Form Validation Library <libraries/form_validation>` rule **valid_url** didn't accept URLs with IPv6 addresses enclosed in square brackets under PHP 5 (upstream bug).
 
 Version 3.0.4
 =============