03. UseYourChainsBuddy

function solve(arg) {
    var pattern = /<p>((?:(?!(?:<\/p>)).)+)+<\/p>/g;
    var output = '';

    for (var i = 0; i < arg.length; i++) {
        while (match = pattern.exec(arg[i])) {
            var str = match[1].replace(/[^a-z\d]+/g, ' ');
            for (var c = 0; c < str.length; c++) {
                var char = str[c];
                if (isLetter(str[c])) {
                    var charCode = str.charCodeAt(c);
                    charCode = charCode < 110 ? charCode += 13 : charCode -= 13;
                    char = String.fromCharCode(charCode);
                }
                output += char;
            }
        }
    }

    console.log(output);

    function isLetter(str) {
        return str.length === 1 && str.match(/[a-z]/g);
    }
}