en.javascript.info/9-regular-expressions/10-regexp-greedy-and-lazy/3-find-html-comments/solution.md

We need to find the beginning of the comment match:<!--, then everything till the end of match:-->.

An acceptable variant is pattern:<!--.*?--> -- the lazy quantifier makes the dot stop right before match:-->. We also need to add flag pattern:s for the dot to include newlines.

Otherwise multiline comments won't be found:

let regexp = /<!--.*?-->/gs;

let str = `... <!-- My -- comment
 test --> ..  <!----> ..
`;

alert( str.match(regexp) ); // '<!-- My -- comment \n test -->', '<!---->'