Trimming a string in a JSTL expression

I have the following line in a JSP page:

<c:if test="${postAuditBean.postAudit.authority == authority.value}">

      

        
        
        
      

    

I would like to change this comparison to first trims the leading and trailing spaces from both the left and right expression before the comparison ... as in something like this:

<c:if test="${trim(postAuditBean.postAudit.authority) == trim(authority.value)}">

      

        
        
        
      

    

I've tried this:

<c:if test="${fn:trim(postAuditBean.postAudit.authority) == fn:trim(authority.value)}">

      

        
        
        
      

    

which generated a parsing error on the JSP page.

Can anyone tell me how to trim whitespace from JSTL subexpressions?