diff --git a/td/telegram/ContactsManager.cpp b/td/telegram/ContactsManager.cpp
index eb94309d3..0c00ba433 100644
--- a/td/telegram/ContactsManager.cpp
+++ b/td/telegram/ContactsManager.cpp
@@ -5353,7 +5353,7 @@ void ContactsManager::check_dialog_username(DialogId dialog_id, const string &us
   if (username.empty()) {
     return promise.set_value(CheckDialogUsernameResult::Ok);
   }
-  if (!is_valid_username(username)) {
+  if (!is_allowed_username(username)) {
     return promise.set_value(CheckDialogUsernameResult::Invalid);
   }
 
@@ -5409,29 +5409,18 @@ td_api::object_ptr<td_api::CheckChatUsernameResult> ContactsManager::get_check_c
   }
 }
 
-bool ContactsManager::is_valid_username(const string &username) {
-  if (username.size() < 5 || username.size() > 32) {
+bool ContactsManager::is_allowed_username(const string &username) {
+  if (!is_valid_username(username)) {
     return false;
   }
-  if (!is_alpha(username[0])) {
+  if (username.size() < 5) {
     return false;
   }
-  for (auto c : username) {
-    if (!is_alpha(c) && !is_digit(c) && c != '_') {
-      return false;
-    }
-  }
-  if (username.back() == '_') {
-    return false;
-  }
-  for (size_t i = 1; i < username.size(); i++) {
-    if (username[i - 1] == '_' && username[i] == '_') {
-      return false;
-    }
-  }
-  if (username.find("admin") == 0 || username.find("telegram") == 0 || username.find("support") == 0 ||
-      username.find("security") == 0 || username.find("settings") == 0 || username.find("contacts") == 0 ||
-      username.find("service") == 0 || username.find("telegraph") == 0) {
+  auto username_lowered = to_lower(username);
+  if (username_lowered.find("admin") == 0 || username_lowered.find("telegram") == 0 ||
+      username_lowered.find("support") == 0 || username_lowered.find("security") == 0 ||
+      username_lowered.find("settings") == 0 || username_lowered.find("contacts") == 0 ||
+      username_lowered.find("service") == 0 || username_lowered.find("telegraph") == 0) {
     return false;
   }
   return true;
@@ -6652,7 +6641,7 @@ void ContactsManager::on_update_profile_success(int32 flags, const string &first
 }
 
 void ContactsManager::set_username(const string &username, Promise<Unit> &&promise) {
-  if (!username.empty() && !is_valid_username(username)) {
+  if (!username.empty() && !is_allowed_username(username)) {
     return promise.set_error(Status::Error(400, "Username is invalid"));
   }
   td_->create_handler<UpdateUsernameQuery>(std::move(promise))->send(username);
@@ -6706,7 +6695,7 @@ void ContactsManager::set_channel_username(ChannelId channel_id, const string &u
     return promise.set_error(Status::Error(400, "Not enough rights to change supergroup username"));
   }
 
-  if (!username.empty() && !is_valid_username(username)) {
+  if (!username.empty() && !is_allowed_username(username)) {
     return promise.set_error(Status::Error(400, "Username is invalid"));
   }
 
diff --git a/td/telegram/ContactsManager.h b/td/telegram/ContactsManager.h
index 83875a9be..885e445fd 100644
--- a/td/telegram/ContactsManager.h
+++ b/td/telegram/ContactsManager.h
@@ -1258,7 +1258,7 @@ class ContactsManager final : public Actor {
 
   void set_my_id(UserId my_id);
 
-  static bool is_valid_username(const string &username);
+  static bool is_allowed_username(const string &username);
 
   void on_set_emoji_status(EmojiStatus emoji_status, Promise<Unit> &&promise);
 
diff --git a/td/telegram/LinkManager.cpp b/td/telegram/LinkManager.cpp
index ee021d33a..e7606ef5e 100644
--- a/td/telegram/LinkManager.cpp
+++ b/td/telegram/LinkManager.cpp
@@ -45,30 +45,6 @@ static bool is_valid_start_parameter(Slice start_parameter) {
   return start_parameter.size() <= 64 && is_base64url_characters(start_parameter);
 }
 
-static bool is_valid_username(Slice username) {
-  if (username.empty() || username.size() > 32) {
-    return false;
-  }
-  if (!is_alpha(username[0])) {
-    return false;
-  }
-  for (auto c : username) {
-    if (!is_alpha(c) && !is_digit(c) && c != '_') {
-      return false;
-    }
-  }
-  if (username.back() == '_') {
-    return false;
-  }
-  for (size_t i = 1; i < username.size(); i++) {
-    if (username[i - 1] == '_' && username[i] == '_') {
-      return false;
-    }
-  }
-
-  return true;
-}
-
 static bool is_valid_phone_number(Slice phone_number) {
   if (phone_number.empty() || phone_number.size() > 32) {
     return false;
diff --git a/td/telegram/misc.cpp b/td/telegram/misc.cpp
index cdd15ba79..7983ed64a 100644
--- a/td/telegram/misc.cpp
+++ b/td/telegram/misc.cpp
@@ -243,6 +243,30 @@ bool is_empty_string(const string &str) {
   return strip_empty_characters(str, str.size()).empty();
 }
 
+bool is_valid_username(Slice username) {
+  if (username.empty() || username.size() > 32) {
+    return false;
+  }
+  if (!is_alpha(username[0])) {
+    return false;
+  }
+  for (auto c : username) {
+    if (!is_alpha(c) && !is_digit(c) && c != '_') {
+      return false;
+    }
+  }
+  if (username.back() == '_') {
+    return false;
+  }
+  for (size_t i = 1; i < username.size(); i++) {
+    if (username[i - 1] == '_' && username[i] == '_') {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 int64 get_vector_hash(const vector<uint64> &numbers) {
   uint64 acc = 0;
   for (auto number : numbers) {
diff --git a/td/telegram/misc.h b/td/telegram/misc.h
index b56bbf41b..934338f6d 100644
--- a/td/telegram/misc.h
+++ b/td/telegram/misc.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "td/utils/common.h"
+#include "td/utils/Slice.h"
 
 namespace td {
 
@@ -31,6 +32,9 @@ string strip_empty_characters(string str, size_t max_length, bool strip_rtlo = f
 // checks if string is empty after strip_empty_characters
 bool is_empty_string(const string &str) TD_WARN_UNUSED_RESULT;
 
+// checks whether a string could be a valid username
+bool is_valid_username(Slice username);
+
 // calculates hash of list of uint64
 int64 get_vector_hash(const vector<uint64> &numbers) TD_WARN_UNUSED_RESULT;