diff --git a/phpBB/includes/functions_post.php b/phpBB/includes/functions_post.php
index 82ff42c64c..f95b835ab2 100644
--- a/phpBB/includes/functions_post.php
+++ b/phpBB/includes/functions_post.php
@@ -81,9 +81,9 @@ function prepare_message($message, $html_on, $bbcode_on, $smile_on, $bbcode_uid
 				for ($i = 0; $i < sizeof($allowed_html_tags); $i++)
 				{
 					$match_tag = trim($allowed_html_tags[$i]);
-					if (preg_match('#^<\/?' . $match_tag . '[> ]#i', $hold_string))
+					if (preg_match('#^<\/?' . $match_tag . '[> ]#is', $hold_string))
 					{
-						$tagallowed = (preg_match('#^<\/?' . $match_tag . ' .*?(style[\t ]*?=|on[\w]+[\t ]*?=)#i', $hold_string)) ? false : true;
+						$tagallowed = (preg_match('#^<\/?' . $match_tag . ' .*?(style[\t ]*?=|on[\w]+[\t ]*?=)#is', $hold_string)) ? false : true;
 					}
 				}