[Zope] Build-in functions

Trying to use the build-in python string functions like replace without success. I want to replace the slash with a dot in an URL (main/detail/form/index=> main.detail.form.index), but <dtml-let g="main/detail/form/index"> <dtml-var replace(g,'/','.')> </dtml-let> doesn't what i expect. It tries to call the object "replace(main/detail/form/index,'/','.')". Do i have to import the build-in functions? How? What's the way doing the replacement? with regards Axel