What is the easiest way to find out character repetition in a string using JS?

I have a string consist of 7 digits, I need a function that returns "true" if the string created by repetition of just 2 digits, something like :

check('9669669');  // true
check('0000001');  // true
check('5555555');  // false
check('1111123');  // false

I want to know the easiest way. thanks.