Bug fix for #4643: Regex object needs to be reset between calls in toolt...

Tooltip.last_token_re is a js regex object that stores its results internally after an exec (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec). When it was starting at the result's lastIndex, it returned null (every other call) before resetting itself. This fixes the bug.