Chris McDonough said:

<dtml-var "mystring" url_quote>

or

&dtml.url_quote-myattr;

You could also use this: <dtml-var "mystring" url_quote_plus> 'url_quote_plus' -- like url_quote but also replace blank space characters with '+'. This is needed for building query strings in some cases. ( in [zope root]/lib/python/DocumentTemplate/DT_Var.py ) @841, Nbk