I'm trying to use regex inside if. Strange, but it doesn't work. How it may be fixed?

It works:

var lang = 'lang-js'; if (lang == 'lang-js') { alert('ok'); } 

It works too (just for testing purposes):

var lang = 'lang-js'; if (lang == 'lang-' + 'js') { alert('ok'); } 

But this one doesn't work:

var lang = 'lang-js'; if (lang == 'lang-' + /[a-z]/) { alert('not ok'); }