`[kanji]{kana}` furigana syntax to HTML <ruby> markup, would've been nice as a Markdown extension.

furigana syntax to HTML ruby markup.js

(string=>{
	string=string.replace(/(\[[^\]]+\]{[^{]+})+/g,x=>"<ruby>"+x+"</ruby>");
	var multiReplace=(s,o)=>{
		for(var key in o){ //What if there were collisions (replacing what's already been replaced)?
			s=s.replace(key,o[key]);
		}
		return s;
	},matches=[...string.matchAll(/\[[^\]]+\]{[^{]+}/g)],iShift=0;
	for(var i=0;i<matches.length;i++){
		var index=matches[i].index,length=matches[i][0].length,
		    newStr=multiReplace(string.substring(iShift+index,iShift+index+length),{"[":"<rb>","]":"</rb>","{":"<rt>","}":"</rt>"});
		string=string.substring(0,iShift+index)+newStr+string.substring(iShift+index+length);
		iShift+=newStr.length-length;
	}
	return string;
})("これは[日]{に}[本]{ほん}[語]{ご}です。"); //'これは<ruby><rb>日</rb><rt>に</rt><rb>本</rb><rt>ほん</rt><rb>語</rb><rt>ご</rt></ruby>です。'