J2EE deployment best practise

I have a J2EE environment having the following modules

1) EJBModule - EJB Project
2) UtilityModule - Java Project
3) WebModule - Struts Project - more than one
4) An EAR Application

All (1), (2) and (3) are added to the EAR Application. There is no error during deployment. However, i keep encountering noClassDefFound error during runtime.

there are some 3rd party jar files used in the UtilityModule (3).
e.g. i wrote a class as follows:
public class Common {
  public String replace(String key, String pattern, String value) {
  org.apache.regexp.RE re = new org.apache.regexp.RE(pattern);
  return re.subst(key, value);      
  }
}

the org.apache.regexp.RE is a class package in a jar that is placed in the classpath of the UtilityModule.

Below are cases where i encounter the problem:
1) Call the replace method in struts project
2) Call the replace method in the EJB module. but access EJB module from struts project.

error still occur if i were to put the 3rd party jar files in the classpath of my struts project.

Can somebody please tell me what's the best practise to arrange my modules to avoid the noclassdeffound exception.