diff --git a/components/org.wso2.carbon.identity.recovery/src/main/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManager.java b/components/org.wso2.carbon.identity.recovery/src/main/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManager.java
index 6577ac269..56aa56fe3 100644
--- a/components/org.wso2.carbon.identity.recovery/src/main/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManager.java
+++ b/components/org.wso2.carbon.identity.recovery/src/main/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManager.java
@@ -1642,6 +1642,10 @@ private boolean validateAgainstRegex(String userName, RealmConfiguration realmCo
 
         String regularExpression = realmConfig
                 .getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG_EX);
+        if (regularExpression == null && !realmConfig.isPrimary()) {
+            regularExpression = realmConfig.getUserStoreProperty(
+                    UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG);
+        }
 
         if (MultitenantUtils.isEmailUserName()) {
             regularExpression = realmConfig
diff --git a/components/org.wso2.carbon.identity.recovery/src/test/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManagerTest.java b/components/org.wso2.carbon.identity.recovery/src/test/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManagerTest.java
index 9d50cf949..639d0cbf9 100644
--- a/components/org.wso2.carbon.identity.recovery/src/test/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManagerTest.java
+++ b/components/org.wso2.carbon.identity.recovery/src/test/java/org/wso2/carbon/identity/recovery/signup/UserSelfRegistrationManagerTest.java
@@ -236,7 +236,9 @@ public class UserSelfRegistrationManagerTest {
     private final String TEST_TENANT_DOMAIN_NAME = "carbon.super";
     private final int TEST_TENANT_ID = 12;
     private final String TEST_USERSTORE_DOMAIN = "PRIMARY";
+    private final String TEST_SECONDARY_USERSTORE_DOMAIN = "SECONDARY";
     private final String TEST_USER_NAME = "dummyUser";
+    private final String TEST_INVALID_USER_NAME = "IS";
     private final String TEST_CLAIM_URI = "ttp://wso2.org/claims/emailaddress";
     private final String TEST_CLAIM_VALUE = "dummyuser@wso2.com";
     private final String TEST_MOBILE_CLAIM_VALUE = "0775553443";
@@ -1429,6 +1431,7 @@ public void testIsMatchUserNameRegex()
         mockedUserCoreUtil.when(() -> UserCoreUtil.removeDomainFromName(anyString())).thenReturn(TEST_USER_NAME);
         mockedIdentityUtil.when(() -> IdentityUtil.extractDomainFromName(anyString()))
                 .thenReturn(TEST_USERSTORE_DOMAIN);
+        when(realmConfiguration.isPrimary()).thenReturn(true);
         when(realmConfiguration.getTenantId()).thenReturn(TEST_TENANT_ID);
         mockedIdentityUtil.when(() -> IdentityTenantUtil.getTenantDomain(TEST_TENANT_ID))
                 .thenReturn(TEST_TENANT_DOMAIN_NAME);
@@ -1438,10 +1441,16 @@ public void testIsMatchUserNameRegex()
         when(validationConfiguration.getField()).thenReturn("username");
         mockedIdentityUtil.when(() -> IdentityUtil.getProperty(Constants.INPUT_VALIDATION_USERNAME_ENABLED_CONFIG))
                 .thenReturn("false");
+        when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG_EX))
+                .thenReturn(null);
+        boolean isMatchUsernameRegex =
+                userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_USER_NAME);
+        assertTrue(isMatchUsernameRegex);
+
         when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG_EX))
                 .thenReturn("^[\\S]{5,30}$");
 
-        boolean isMatchUsernameRegex =
+        isMatchUsernameRegex =
                 userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_USER_NAME);
         assertTrue(isMatchUsernameRegex);
 
@@ -1486,6 +1495,62 @@ public void testIsMatchUserNameRegex()
         }
     }
 
+    @Test
+    public void testIsMatchUserNameRegexForSecondaryUserStore()
+            throws IdentityRecoveryException, InputValidationMgtException,
+            UserStoreException {
+
+        mockedMultiTenantUtils.when(() -> MultitenantUtils
+                .getTenantAwareUsername(eq(TEST_USER_NAME))).thenReturn(TEST_USER_NAME);
+        mockedUserCoreUtil.when(() -> UserCoreUtil.removeDomainFromName(eq(TEST_USER_NAME))).thenReturn(TEST_USER_NAME);
+        mockedIdentityUtil.when(() -> IdentityUtil.extractDomainFromName(anyString()))
+                .thenReturn(TEST_SECONDARY_USERSTORE_DOMAIN);
+        when(realmConfiguration.getTenantId()).thenReturn(TEST_TENANT_ID);
+        mockedIdentityUtil.when(() -> IdentityTenantUtil.getTenantDomain(TEST_TENANT_ID))
+                .thenReturn(TEST_TENANT_DOMAIN_NAME);
+
+        when(inputValidationManagementService.getInputValidationConfiguration(anyString()))
+                .thenReturn(Arrays.asList(validationConfiguration));
+        when(validationConfiguration.getField()).thenReturn("username");
+        mockedIdentityUtil.when(() -> IdentityUtil.getProperty(Constants.INPUT_VALIDATION_USERNAME_ENABLED_CONFIG))
+                .thenReturn("false");
+        when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG_EX))
+                .thenReturn(null);
+        when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG))
+                .thenReturn("^[\\S]{5,30}$");
+
+        // regex read from fallback key
+        boolean isMatchUsernameRegex =
+                userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_USER_NAME);
+        assertTrue(isMatchUsernameRegex);
+
+        mockedMultiTenantUtils.when(() -> MultitenantUtils
+                .getTenantAwareUsername(eq(TEST_INVALID_USER_NAME))).thenReturn(TEST_INVALID_USER_NAME);
+        mockedUserCoreUtil.when(() -> UserCoreUtil.removeDomainFromName(eq(TEST_INVALID_USER_NAME)))
+                .thenReturn(TEST_INVALID_USER_NAME);
+        isMatchUsernameRegex =
+                userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_INVALID_USER_NAME);
+        assertFalse(isMatchUsernameRegex);
+
+        // regex read main key
+        when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG_EX))
+                .thenReturn("^[\\S]{5,30}$");
+        when(realmConfiguration.getUserStoreProperty(UserCoreConstants.RealmConfig.PROPERTY_USER_NAME_JAVA_REG))
+                .thenReturn(null);
+        isMatchUsernameRegex =
+                userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_USER_NAME);
+        assertTrue(isMatchUsernameRegex);
+
+        mockedMultiTenantUtils.when(() -> MultitenantUtils
+                .getTenantAwareUsername(eq(TEST_INVALID_USER_NAME))).thenReturn(TEST_INVALID_USER_NAME);
+        mockedUserCoreUtil.when(() -> UserCoreUtil.removeDomainFromName(eq(TEST_INVALID_USER_NAME)))
+                .thenReturn(TEST_INVALID_USER_NAME);
+        isMatchUsernameRegex =
+                userSelfRegistrationManager.isMatchUserNameRegex(TEST_TENANT_DOMAIN_NAME, TEST_INVALID_USER_NAME);
+        assertFalse(isMatchUsernameRegex);
+
+    }
+
     @Test
     public void testPreValidatePasswordWithConfirmationKey() throws Exception {