diff --git a/zanata-war/src/main/java/org/zanata/action/RegisterAction.java b/zanata-war/src/main/java/org/zanata/action/RegisterAction.java
index 812da7d1fb..fd6d647355 100644
--- a/zanata-war/src/main/java/org/zanata/action/RegisterAction.java
+++ b/zanata-war/src/main/java/org/zanata/action/RegisterAction.java
@@ -91,7 +91,7 @@ public void setUsername(String username) {
 
     @NotEmpty
     @Size(min = 3, max = 20)
-    @Pattern(regexp = "^[a-z\\d_]{3,20}$",
+    @Pattern(regexp = "^([a-z\\d][a-z\\d_]*){3,20}$",
             message = "{validation.username.constraints}")
     public String getUsername() {
         return username;