量词：*, +, ?, {n}, {n,}, {n,m}

// Greedy
atom?
atom*
atom+
atom{count}
atom{min,}
atom{min,max}

// Non-greedy
atom??
atom*?
atom+?
atom{count}?
atom{min,}?
atom{min,max}?

const re = /a{1, 3}/;
re.test("aa"); // false
re.test("a{1, 3}"); // true

/a{1, 3}/u; // SyntaxError: Invalid regular expression: Incomplete quantifier

/a{3,2}/; // SyntaxError: Invalid regular expression: numbers out of order in {} quantifier

/[ab]*/.exec("aba"); // ['aba']

/a*/.exec("aaa"); // ['aaa']; the entire input is consumed
/a*?/.exec("aaa"); // ['']; it's possible to consume no characters and still match successfully
/^a*?$/.exec("aaa"); // ['aaa']; it's not possible to consume fewer characters and still match successfully

/a*?$/.exec("aaa"); // ['aaa']; the match already succeeds at the first character, so the regex never attempts to start matching at the second character

/[ab]+[abc]c/.exec("abbc"); // ['abbc']

/^*/; // SyntaxError: Invalid regular expression: nothing to repeat

/(?=a)?b/.test("b"); // true; the lookahead is matched 0 time

function stripTags(str) {
  return str.replace(/<.+?>/g, "");
}

stripTags("<p><em>lorem</em> <strong>ipsum</strong></p>"); // 'lorem ipsum'

function stripTags(str) {
  return str.replace(/<[^>]+>/g, "");
}

stripTags("<p><em>lorem</em> <strong>ipsum</strong></p>"); // 'lorem ipsum'

function countParagraphs(str) {
  return str.match(/(?:\r?\n){2,}/g).length + 1;
}

countParagraphs(`
Paragraph 1

Paragraph 2
Containing some line breaks, but still the same paragraph

Another paragraph
`); // 3