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function countDivs(html) {
    console.log(html.match(/<div/g).length);
}

countDivs('<!DOCTYPE html>\n' +
'<html>\n' +
'<head lang="en">\n' +
'<meta charset="UTF-8">\n' +
'<title>index</title>\n' +
'<script src="/yourScript.js" defer></script>\n' +
'</head>\n' +
'<body>\n' +
'<div id="outerDiv">\n' +
'<div\n' +
'class="first">\n' +
'<div><div>hello</div></div>\n' +
'</div>\n' +
'<div>hi<div></div></div>\n' +
'<div>I am a div</div>\n' +
'</div>\n' +
'</body>\n' +
'</html>');