It’s a bug in the RegEx engine, a similar question with the same issue came up here.
From my answer to that question: It’s a bug with the way regexes are implemented in ECMAScript 3, there’s a great post on the details here.
The basics are a /regex/ with the g modifier doesn’t reset correctly, so multiple .test() calls alternate between true and false if everyone should be true, every other calls successfully resets it.