Manipulate Strings with Struts²

I'm working on a Java Web App under Struts², and i'm having trouble using the taglib s:if.

I have a test <s:if>, but instead of writing a test like this

<s:if test="string == myValue">

I'd like to write something like this :

<s:if test="string.contains(myValue)"> , or using a substring function

Does such functions exist ? How to use it ?
Thanks for your help.