diff --git a/README.md b/README.md
index 2d73f81..2359f5a 100644
--- a/README.md
+++ b/README.md
@@ -1718,9 +1718,7 @@ Checks that the class has a `package` definition.
 
 Checks the format of package names.
 
-Identifiers must match `^[a-z]+(\.[a-zA-Z_][a-zA-Z0-9_]*)*$`.
-
-> TODO: format = `^[a-z]+(\.[a-z]+)*$` - only lowercase letters, no numbers or underscores
+Identifiers must match `^[a-z]+(\.[a-z]+)*$`.
 
 #### [ParameterName](http://checkstyle.sourceforge.net/config_naming.html#ParameterName)
 
diff --git a/src/main/resources/net/kemitix/checkstyle.xml b/src/main/resources/net/kemitix/checkstyle.xml
index f56d306..4291089 100644
--- a/src/main/resources/net/kemitix/checkstyle.xml
+++ b/src/main/resources/net/kemitix/checkstyle.xml
@@ -165,7 +165,9 @@
         <module name="OverloadMethodsDeclarationOrder"/> <!-- group overloaded methods together -->
 
         <module name="PackageDeclaration"/> <!-- class must have package and it must match the directory -->
-        <module name="PackageName"/> <!-- validate package name format -->
+        <module name="PackageName"> <!-- validate package name format -->
+            <property name="format" value="^[a-z]+(\.[a-z]+)*$"/> <!-- only lowercase letters, no numbers of underscores -->
+        </module>
         <module name="ParameterName"/> <!-- validate parameter name format -->
         <module name="ParameterNumber"/> <!-- limits the number of parameters to 7 -->
         <module name="ParenPad"/> <!-- parentheses should have no padding spaces -->