diff --git a/WHATSNEW b/WHATSNEW
index 0a05714a5..a45eb99e5 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -37,6 +37,10 @@ Fixed bugs:
  * ExtensionPoint doesn't work with nested import/include
    Bugzilla Report 53405.
 
+ * <packagemapper> failed to strip the non-matched parts with
+   handledirsep="true".
+   Bugzilla Report 53399.
+
 Other changes:
 --------------
 
diff --git a/src/main/org/apache/tools/ant/util/PackageNameMapper.java b/src/main/org/apache/tools/ant/util/PackageNameMapper.java
index 87477c3d9..30256670a 100644
--- a/src/main/org/apache/tools/ant/util/PackageNameMapper.java
+++ b/src/main/org/apache/tools/ant/util/PackageNameMapper.java
@@ -41,7 +41,7 @@ public class PackageNameMapper extends GlobPatternMapper {
         String var = name.substring(prefixLength,
                 name.length() - postfixLength);
         if (getHandleDirSep()) {
-            var = name.replace('/', '.').replace('\\', '.');
+            var = var.replace('/', '.').replace('\\', '.');
         }
         return var.replace(File.separatorChar, '.');
     }
diff --git a/src/tests/antunit/types/mappers/packagemapper-test.xml b/src/tests/antunit/types/mappers/packagemapper-test.xml
index f5cada734..b080e4063 100644
--- a/src/tests/antunit/types/mappers/packagemapper-test.xml
+++ b/src/tests/antunit/types/mappers/packagemapper-test.xml
@@ -31,6 +31,20 @@
       <string>foo\bar</string>
       <packagemapper from="*" to="*" handledirsep="true"/>
     </pathconvert>
+    <au:assertPropertyEquals name="p2" value="foo.bar"/>
+  </target>
+
+  <target name="testMapsOnlySubstring"
+          description="https://issues.apache.org/bugzilla/show_bug.cgi?id=53399">
+    <pathconvert property="p1">
+      <string>xyzzy${file.separator}foo${file.separator}bar</string>
+      <packagemapper from="xyzzy/*" to="*" handledirsep="false"/>
+    </pathconvert>
     <au:assertPropertyEquals name="p1" value="foo.bar"/>
+    <pathconvert property="p2">
+      <string>xyzzy${file.separator}foo${file.separator}bar</string>
+      <packagemapper from="xyzzy/*" to="*" handledirsep="true"/>
+    </pathconvert>
+    <au:assertPropertyEquals name="p2" value="foo.bar"/>
   </target>
 </project>
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