diff --git a/README.md b/README.md
index 9690921..6292a06 100644
--- a/README.md
+++ b/README.md
@@ -1809,9 +1809,9 @@ Checks that the class has a `package` definition.
 
 #### [PackageName](http://checkstyle.sourceforge.net/config_naming.html#PackageName)
 
-Checks the format of package names.
+Checks the format of package names. Only lowercase letters, no initial numbers or any underscores.
 
-Identifiers must match `^[a-z]+(\.[a-z]+)*$`.
+Identifiers must match `^[a-z]+(\.[a-z][a-z0-9]+)*$`.
 
 #### [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 66fa2ab..997c29e 100644
--- a/src/main/resources/net/kemitix/checkstyle.xml
+++ b/src/main/resources/net/kemitix/checkstyle.xml
@@ -180,7 +180,7 @@
         <module name="PackageAnnotation"/> <!-- package level annotations appear in package-info.java -->
         <module name="PackageDeclaration"/> <!-- class must have package and it must match the directory -->
         <module name="PackageName"> <!-- validate package name format -->
-            <property name="format" value="^[a-z]+(\.[a-z]+)*$"/> <!-- only lowercase letters, no numbers of underscores -->
+            <property name="format" value="^[a-z]+(\.[a-z][a-z0-9]+)*$"/> <!-- only lowercase letters, no initial numbers or underscores -->
         </module>
         <module name="ParameterName"/> <!-- validate parameter name format -->
         <module name="ParameterNumber"> <!-- limits the number of parameters to 7 -->