iReport variable expression returns null

Here's the simple code in XML view of print form:

<variable name="TITLE" class="java.lang.String">
     <variableExpression><![CDATA[$F{APP_NUMBER}.replaceAll( "app_", "" )]]></variableExpression>

If I use this variable in form, it returns null (while using "$V{TITLE}"). But if I remove replaceAll method, it works and returns correct value. What is wrong with expression? Should I use something instead of "<variableExpression>"?