JSFiddle - React, Tailwind, and code Playground

JavaScript

var s = '[quote=SoundMAX][quote=Laplundik][quote=SoundMAX]\nblahblahblah[/quote]\nblahblah[/quote]\nblah[/quote]';

var quoteLevel = 0;

alert(s.replace(/\[(\/?)quote[^\]]*\]|./gi, function(tag, slash) {
    // Opening tag?
    if(tag.length > 1 && !slash.length) quoteLevel += 1;
    // What to strip
    var strip = quoteLevel > 2;
    // Closing tag?
    if(tag.length > 1 && slash.length) quoteLevel -= 1;
    
    if(strip) return '';
    return tag;
}));