<?php

$r = '~^(?:,|([^,])(?!\\1))+$~';
var_dump(preg_match($r, 'a,,,sd')); // true - because ,,, is allowed
var_dump(preg_match($r, 'asd')); // true - because no repetitions
var_dump(preg_match($r, 'assd')); // false - because double-s