lightweight MD like parser

md.js

const regex = {
    headline: /^(#{1,6})([^#\n]+)$/m,
    hr: /^(?:([*\-_] ?)+)\1\1$/gm,
    style: /(?:([*_~]{1,3}))([^*_~\n]+[^*_~\s])\1/g
};

export default function(str) {
    let stra;

    /* headlines */
    stra = regex.headline.exec(str);

    while (stra !== null) {
        const count = stra[1].length;

        str = str.replace(stra[0], `<h${count}>${stra[2].trim()}</h${count}>`).trim();
        stra = regex.headline.exec(str);
    }

    /* strikethrough, underline, bold and italic */
    for (let i = 0; i < 4; i++) {
        stra = regex.style.exec(str);

        while (stra !== null) {
            let repstr = [];

            if (stra[1].indexOf('~~') >= 0) {
                str = str.replace(stra[0], `<del>${stra[2]}</del>`);
            } else if (stra[1].indexOf('_') >= 0) {
                str = str.replace(stra[0], `<u>${stra[2]}</u>`);
            } else {
                switch (stra[1].length) {
                    case 1:
                        repstr = ['<i>', '</i>'];
                        break;
                    case 2:
                        repstr = ['<b>', '</b>'];
                        break;
                    case 3:
                        repstr = ['<i><b>', '</b></i>'];
                        break;
                    default:
                        repstr = ['', ''];

                }

                str = str.replace(stra[0], repstr[0] + stra[2] + repstr[1]);
            }

            stra = regex.style.exec(str);
        }
    }

    /* horizontal line */
    stra = regex.hr.exec(str);

    while (stra !== null) {
        str = str.replace(stra[0], '\n<hr/>\n');
        stra = regex.hr.exec(str);
    }

    return str;
};