Well the way I'm using it is like <mas:format_date><mas:foo_date></mas:format_date> so first foo_date is replaced by its value and then the callback function takes the contents and processes it according to preferences.

Another usage is <mas:limit_string max="50">test test</mas:limit_string> it will limit the string length to 50 characters.

I'm having trouble using a single regex to match both though, for some reason a (.*?) before $this->{mask_end} (in this case >) will not match the first example, i.e. when there're no params to pass to the function.