With JavaScript :

/^(.)\1+$/i.test(value)

Works for upper and lower case mix, for a string of at least two characters :

/^(.)\1+$/i.test('a')
false
/^(.)\1+$/i.test('aa')
true
/^(.)\1+$/i.test('AA')
true
/^(.)\1+$/i.test('Aa')
true
/^(.)\1+$/i.test('Abc')
false