Very basic converter just for personal use since I generally write documentation in Markdown and converting to BBCode for SA:MP forums is pretty annoying.

markdown-to-bbcode.js

var fs = require('fs');
var util = require('util');

var
	regexHeader1 = new RegExp('^# (.*)'),
	replcHeader1 = ['[COLOR="FF4700"][SIZE="7"][B]', '[/B][/SIZE][/COLOR]'],

	regexHeader2 = new RegExp('^## (.*)'),
	replcHeader2 = ['[COLOR="RoyalBlue"][SIZE="6"][B]', '[/B][/SIZE][/COLOR]'],

	regexHeader3 = new RegExp('^### (.*)'),
	replcHeader3 = ['[COLOR="DeepSkyBlue"][SIZE="5"][B]', '[/B][/SIZE][/COLOR]'],

	replcItalics = ['[I]', '[/I]'],

	regexBoldAster = new RegExp('\\*\\*'),
	regexBoldUnder = new RegExp('__'),
	replcBold = ['[B]', '[/B]'],
	replcCodeBlock = ['[PAWN]', '[/PAWN]'],

	regexCodeInlineOpen = new RegExp('(```).'),
	regexCodeInlineClose = new RegExp('.(```)'),
	replcCodeInline = ['[FONT="courier new"]', '[/FONT]'];


if(process.argv.length !== 3)
{
	console.error('Usage: markdown-to-bbcode <filename>');

	process.exit(1);
}
else if(!fs.existsSync(process.argv[2]))
{
	console.error('Error: Invalid filename given');

	process.exit(1);
}
else
{
	var
		line = fs.readFileSync(process.argv[2], { encoding: 'utf-8' }),
		output = line.split('\r\n');

	var
		openItalics = false,
		openBold = false,
		openCodeInline = false,
		openCodeBlock = false;

	for (var i = 0; i < output.length; i++)
	{
		var match = regexHeader1.exec(output[i]);

		if(match != null)
		{
			output[i] = replcHeader1[0]+match[1]+replcHeader1[1];
			continue;
		}

		var match = regexHeader2.exec(output[i]);

		if(match != null)
		{
			output[i] = replcHeader2[0]+match[1]+replcHeader2[1];
			continue;
		}

		var match = regexHeader3.exec(output[i]);

		if(match != null)
		{
			output[i] = replcHeader3[0]+match[1]+replcHeader3[1];
			continue;
		}

		if(output[i] == "```")
		{
			if(openCodeBlock == false)
			{
				output[i] = replcCodeBlock[0];
				openCodeBlock = true;
			}
			else
			{
				output[i] = replcCodeBlock[1];
				openCodeBlock = false;
			}
		}

		for (var j = 0; j < output[i].length; j++)
		{
			if(openCodeInline == false)
			{
				var match = regexCodeInlineOpen.exec(output[i]);

				if(match != null)
				{
					var pos = output[i].search(match[1]);

					output[i] = output[i].slice(0, pos) + replcCodeInline[0] + output[i].slice(pos + match[1].length);

					j += replcCodeInline[0].length;
					openCodeInline = true;

					continue;
				}
			}
			else
			{
				var match = regexCodeInlineClose.exec(output[i]);

				if(match != null)
				{
					var pos = output[i].search(match[1]);

					output[i] = output[i].slice(0, pos) + replcCodeInline[1] + output[i].slice(pos + match[1].length);

					j += replcCodeInline[1].length;
					openCodeInline = true;
				}

				continue;
			}

			if(openBold == false)
			{
				var match = regexBoldAster.exec(output[i]);

				if(match == null)
					match = regexBoldUnder.exec(output[i]);

				if(match != null)
				{
					var pos = output[i].search(regexBoldAster);

					output[i] = output[i].slice(0, pos) + replcBold[0] + output[i].slice(pos + 2);

					j += replcBold[0].length;
					openBold = true;

					continue;
				}
			}
			else
			{
				var match = regexBoldAster.exec(output[i]);

				if(match == null)
					match = regexBoldUnder.exec(output[i]);

				if(match != null)
				{
					var pos = output[i].search(regexBoldAster);

					output[i] = output[i].slice(0, pos) + replcBold[1] + output[i].slice(pos + 2);

					j += replcBold[1].length;
					openBold = false;

					continue;
				}
			}

			if(output[i][j] == '*' && output[i][j + 1] != '*' && !openCodeBlock)
			{
				if(j > 0 && output[i][j - 1] == '*')
					continue;

				if(openItalics == false)
				{
					output[i] = output[i].slice(0, j) + replcItalics[0] + output[i].slice(j+1);
					j += replcItalics[0].length-1;
					openItalics = true;
				}
				else
				{
					output[i] = output[i].slice(0, j) + replcItalics[1] + output[i].slice(j+1);
					j += replcItalics[1].length-1;
					openItalics = false;
				}
			}

			if(output[i][j] == '_' && output[i][j + 1] != '_' && !openCodeBlock)
			{
				if(j > 0 && output[i][j - 1] == '_')
					continue;

				if(openItalics == false)
				{
					output[i] = output[i].slice(0, j) + replcItalics[0] + output[i].slice(j+1);
					j += replcItalics[0].length-1;
					openItalics = true;
				}
				else
				{
					output[i] = output[i].slice(0, j) + replcItalics[1] + output[i].slice(j+1);
					j += replcItalics[1].length-1;
					openItalics = false;
				}
			}
/*
			if(output[i][j] == '_')
			{
				if(openItalics == false)
				{
					output[i] = output[i].slice(0, j) + replcItalics[0] + output[i].slice(j);
					openItalics = true;
				}
				else
				{
					output[i] = output[i].slice(0, j) + replcItalics[1] + output[i].slice(j);
					openItalics = false;
				}
			}*/
		};
		console.log(output[i]);
	};

	fs.writeFile("output.txt", output.join("\r\n"), function(err)
	{
		if(err)
		{
			console.log(err);
		}
		else
		{
			console.log("The file was saved!");
		}
	});
}