Crowded's simple but bulletproof markdown filter

simple-md.js

/**
Copyright (c) 2017 Crowded. All rights reserved.

This work is licensed under the terms of the MIT license.  
For a copy, see <https://opensource.org/licenses/MIT>.
**/

function simpleMd(text) {
    var text = String(text);

    text = text.replace(/\\`/g, '&grave;').replace(/\\\*/g, '&ast;').replace(/\\\_/g, '&lowbar;').replace(/\\\~/g, '&#x7E;'); // first html entitize escaped vars

    //code block ``` ... ``` 
    text = text.replace(/^```$([\s\S]*?)^```$/gm, function(s){ return '<code>'+s.substring(4, s.length-4)+'</code>' })

    //list
    text = text.replace(/(^\*\ .*?([\n\r]|$))+/gm, function(s){                 
        var list = '<ul>'
        list += s.replace(/^\*\ .*?$/gm, function(s){ return '<li>'+s.substring(2)+'</li>' })
        list += '</ul>'
        return list;
    })

    // for inline, first match based on whitespace or string beginning/end, then replace the matched part

    //inline code ` ... `
    text = text.replace(/(\s|^|[_*~>])`([^\r\n]*?)`(\s|$|[_*~<,.])/gm, function(s){ return s.replace(/`([^\r\n]*?)`(?!`)/gm, function(s){ return '<span class="inline-code">'+s.substring(1, s.length-1)+'</span>' }) })

    //strong * ... *
    text = text.replace(/(\s|^|[_~`>])\*([^\r\n]*?)\*(\s|$|[_~`<,.])/gm, function(s){ return s.replace(/\*([^\r\n]*?)\*(?!\*)/gm, function(s){ return '<strong>'+s.substring(1, s.length-1)+'</strong>' }) })

    //italic _ ... _
    text = text.replace(/(\s|^|[*~`>])\_([^\r\n]*?)\_(\s|$|[*~`<,.])/gm, function(s){ return s.replace(/\_([^\r\n]*?)\_(?!\_)/gm, function(s){ return '<em>'+s.substring(1, s.length-1)+'</em>' }) })

    //strikethrough ~ ... ~
    text = text.replace(/(\s|^|[_*`>])\~([^\r\n]*?)\~(\s|$|[_*`<,.])/gm, function(s){ return s.replace(/\~([^\r\n]*?)\~(?!\~)/gm, function(s){ return '<strike>'+s.substring(1, s.length-1)+'</strike>' }) })

    return text;
};