s/\s+/:/g
[download]

or

s/[ ]+/:/g
[download]

\s = space-like things, like tabs
+ = one or more
[ ] = just a normal space

You might want to have a look at perldoc perlre. What you have matches anything (".") twice and then one or more spaces, replacing them all with a colon.

--
Allolex

Update: I updated this page a whole lot of times, adding more and more info each time. Silly, isn't it?