JSFiddle - React, Tailwind, and code Playground

JavaScript

let parenth = 0;
const str = `thisIsA100CharactersLong:SampleStringContaining-SpecialChar[1-300]10&20*`,
  spaced = str.replace(/[A-Z]|\d+|\W/g, (m) => {
		if (/[({[]/.test(m)) {
   		parenth = 1;
     	return m;
   	}
		if (parenth || /[)}\]]/.test(m)) {
    	parenth = 0;
      return m;
    }
    return ` ${m}`;    
	});
console.log(spaced);