diff --git a/phpBB/docs/CHANGELOG.html b/phpBB/docs/CHANGELOG.html
index 1c958e2594..8509b875ef 100644
--- a/phpBB/docs/CHANGELOG.html
+++ b/phpBB/docs/CHANGELOG.html
@@ -109,6 +109,7 @@
 		<li>[Fix] Deleting all posts in a topic - bad redirect (Bug #41705 - Patch by TerraFrost)</li>
 		<li>[Fix] Deleted users still appear logged in (Bug #41985 - Patch by TerraFrost)</li>
 		<li>[Fix] Removed redundant code and unnecessary queries in forum management. (Bug #42265 - Patch by nickvergessen)</li>
+		<li>[Fix] Correct mbstring regular expression for the allowable username characters, only affects <code>USERNAME_LETTER_NUM_SPACERS</code>. (Bug #42325)</li>
 		<li>[Change] Allow download of conflicting file for later reference in automatic updater</li>
 		<li>[Change] Default difference view is now 'inline' instead of 'side by side'</li>
 		<li>[Change] Added new option for merging differences to conflicting files in automatic updater</li>
diff --git a/phpBB/includes/functions_user.php b/phpBB/includes/functions_user.php
index 3cdf18449d..f8589b274f 100644
--- a/phpBB/includes/functions_user.php
+++ b/phpBB/includes/functions_user.php
@@ -1436,7 +1436,7 @@ function validate_username($username, $allowed_username = false)
 			}
 			else if ($mbstring)
 			{
-				$regex = '[-\]_+ [[:upper:][:lower:][:digit:]]+';
+				$regex = '[-\]_+ \[[:upper:][:lower:][:digit:]]+';
 			}
 			else
 			{