diff --git a/wp-includes/class-phpmailer.php b/wp-includes/class-phpmailer.php
index a7ad292db8..25e5570a3f 100644
--- a/wp-includes/class-phpmailer.php
+++ b/wp-includes/class-phpmailer.php
@@ -1409,7 +1409,7 @@ class PHPMailer {
    */
   function EncodeQ ($str, $position = 'text') {
     /* There should not be any EOL in the string */
-    $encoded = preg_replace("[\r\n]", '', $str);
+    $encoded = preg_replace("/[\r\n]/", '', $str);
 
     switch (strtolower($position)) {
       case 'phrase':
diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index ab7b4c176a..166dc5294d 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -1960,7 +1960,7 @@ function attribute_escape( $text ) {
  * @return string
  */
 function tag_escape($tag_name) {
-	$safe_tag = strtolower( preg_replace('[^a-zA-Z_:]', '', $tag_name) );
+	$safe_tag = strtolower( preg_replace('/[^a-zA-Z_:]/', '', $tag_name) );
 	return apply_filters('tag_escape', $safe_tag, $tag_name);
 }
 
diff --git a/wp-includes/post-template.php b/wp-includes/post-template.php
index 8db1f196b0..ae44aa2a2a 100644
--- a/wp-includes/post-template.php
+++ b/wp-includes/post-template.php
@@ -607,7 +607,7 @@ function wp_list_pages($args = '') {
 	$current_page = 0;
 
 	// sanitize, mostly to keep spaces out
-	$r['exclude'] = preg_replace('[^0-9,]', '', $r['exclude']);
+	$r['exclude'] = preg_replace('/[^0-9,]/', '', $r['exclude']);
 
 	// Allow plugins to filter an array of excluded pages
 	$r['exclude'] = implode(',', apply_filters('wp_list_pages_excludes', explode(',', $r['exclude'])));