Making built-in completion recognize : as word boundary

["Matthias B." <msbREMOVE-THIS@xxxxxxxxxxxxxxx>]

Something that's been bothering me for years now with zsh is that the
built-in completion can't complete

PATH=/usr/bin:/sb<TAB>

presumably because it doesn't consider the colon a word boundary. Could
someone please point me at the code responsible so that I can fix this.

MSB

p.s.: I know that the function-based completion system doesn't have this
problem, but I don't want to use it.

-- 
2+2=5, for moderately large values of two.