What is meaning of [_|\_|\.]? in Javascript regexps?

What is meaning of [_|\_|\.]? in Javascript regexps?

I have a js codeF
/^([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+@([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+\.[a-zA-Z]{2,3}$/
But what's meaning of [_|\_|\.]?ijs regexpj