diff --git a/src/main/java/stirling/software/SPDF/config/security/UserService.java b/src/main/java/stirling/software/SPDF/config/security/UserService.java
index 2bc93352..eefa5a2c 100644
--- a/src/main/java/stirling/software/SPDF/config/security/UserService.java
+++ b/src/main/java/stirling/software/SPDF/config/security/UserService.java
@@ -329,12 +329,16 @@ public class UserService implements UserServiceInterface {
 
     public boolean isUsernameValid(String username) {
         // Checks whether the simple username is formatted correctly
+        // Regular expression for user name: Min. 3 characters, max. 50 characters
         boolean isValidSimpleUsername =
-                username.matches("^[a-zA-Z0-9][a-zA-Z0-9@._+-]*[a-zA-Z0-9]$");
+                username.matches("^[a-zA-Z0-9](?!.*[-@._+]{2,})[a-zA-Z0-9@._+-]{1,48}[a-zA-Z0-9]$");
+
         // Checks whether the email address is formatted correctly
+        // Regular expression for email addresses: Max. 320 characters, with RFC-like validation
         boolean isValidEmail =
                 username.matches(
-                        "^(?=.{1,64}@)[A-Za-z0-9]+(\\.[A-Za-z0-9_+.-]+)*@[^-][A-Za-z0-9-]+(\\.[A-Za-z0-9-]+)*(\\.[A-Za-z]{2,})$");
+                        "^(?=.{1,320}$)(?=.{1,64}@)[A-Za-z0-9](?:[A-Za-z0-9_.+-]*[A-Za-z0-9])?@[^-][A-Za-z0-9-]+(?:\\\\.[A-Za-z0-9-]+)*(?:\\\\.[A-Za-z]{2,})$");
+
         List<String> notAllowedUserList = new ArrayList<>();
         notAllowedUserList.add("ALL_USERS".toLowerCase());
         boolean notAllowedUser = notAllowedUserList.contains(username.toLowerCase());
diff --git a/src/main/resources/templates/account.html b/src/main/resources/templates/account.html
index 0fa36504..e054bdbd 100644
--- a/src/main/resources/templates/account.html
+++ b/src/main/resources/templates/account.html
@@ -104,7 +104,14 @@
               </div>
               <script th:inline="javascript">
                 jQuery.validator.addMethod("usernamePattern", function(value, element) {
-                    return this.optional(element) || /^[a-zA-Z0-9][a-zA-Z0-9@._+-]*[a-zA-Z0-9]$|^(?=.{1,64}@)[A-Za-z0-9]+(\.[A-Za-z0-9_+.-]+)*@[^-][A-Za-z0-9-]+(\.[A-Za-z0-9-]+)*(\.[A-Za-z]{2,})$/.test(value);
+                  // Regular expression for user name: Min. 3 characters, max. 50 characters
+                  const regexUsername = /^[a-zA-Z0-9](?!.*[-@._+]{2,})([a-zA-Z0-9@._+-]{1,48})[a-zA-Z0-9]$/;
+
+                  // Regular expression for email addresses: Max. 320 characters, with RFC-like validation
+                  const regexEmail = /^(?=.{1,320}$)(?=.{1,64}@)[A-Za-z0-9](?:[A-Za-z0-9_.+-]*[A-Za-z0-9])?@[^-][A-Za-z0-9-]+(?:\.[A-Za-z0-9-]+)*(?:\.[A-Za-z]{2,})$/;
+
+                  // Check if the field is optional or meets the requirements
+                  return this.optional(element) || regexUsername.test(value) || regexEmail.test(value);
                 }, /*[[#{invalidUsernameMessage}]]*/ "Invalid username format");
                 $(document).ready(function() {
                   $.validator.addMethod("passwordMatch", function(value, element) {
diff --git a/src/main/resources/templates/addUsers.html b/src/main/resources/templates/addUsers.html
index 088b7117..9b6475f3 100644
--- a/src/main/resources/templates/addUsers.html
+++ b/src/main/resources/templates/addUsers.html
@@ -207,7 +207,14 @@
 
       <script th:inline="javascript">
         jQuery.validator.addMethod("usernamePattern", function(value, element) {
-            return this.optional(element) || /^[a-zA-Z0-9][a-zA-Z0-9@._+-]*[a-zA-Z0-9]$|^(?=.{1,64}@)[A-Za-z0-9]+(\.[A-Za-z0-9_+.-]+)*@[^-][A-Za-z0-9-]+(\.[A-Za-z0-9-]+)*(\.[A-Za-z]{2,})$/.test(value);
+          // Regular expression for user name: Min. 3 characters, max. 50 characters
+          const regexUsername = /^[a-zA-Z0-9](?!.*[-@._+]{2,})([a-zA-Z0-9@._+-]{1,48})[a-zA-Z0-9]$/;
+
+          // Regular expression for email addresses: Max. 320 characters, with RFC-like validation
+          const regexEmail = /^(?=.{1,320}$)(?=.{1,64}@)[A-Za-z0-9](?:[A-Za-z0-9_.+-]*[A-Za-z0-9])?@[^-][A-Za-z0-9-]+(?:\.[A-Za-z0-9-]+)*(?:\.[A-Za-z]{2,})$/;
+
+          // Check if the field is optional or meets the requirements
+          return this.optional(element) || regexUsername.test(value) || regexEmail.test(value);
         }, /*[[#{invalidUsernameMessage}]]*/ "Invalid username format");
         $(document).ready(function() {
           $('[data-toggle="tooltip"]').tooltip();