function myFunction() {
var source
= '<p class="boid main-root jss439">10日</p>'
+ ':略'
+ '<p class="boid main-root jss569">18日</p>'
+ ':略'
+ '<p class="boid main-root jss659">20日</p>'
;
const regex = /jss\d{3}">(.*?)</g;
const days = [];
var match;
while ((match = regex.exec(source)) !== null) days.push(match[1]);
Logger.log(days);

}