To access the groups, you will need to use .exec()
repeatedly:
var regex = /(alpha)|(beta)|(gamma)/gi,
str = "Betamax. Digamma. Alphabet. Hebetation.";
for (var nums = [], match; match = regex.exec(str); )
nums.push(match.lastIndexOf(match[0]));
If you want the indizes zero-based, you could use
nums.push(match.slice(1).indexOf(match[0]));