dashes vs underscores in URL

naming-conventions, url-rewriting

Solution

use dashes

read from the matt's blog http://www.mattcutts.com/blog/dashes-vs-underscores/

Problem

In URL rewriting, i am quite confused that should i use underscore (_) OR hyphen (-) to replace spaces. According to this and this, hyphen should be used. But wikipedia uses underscores like en.wikipedia.org/wiki/Computer_network_programming. so which should be used?

Original source