JSFiddle - React, Tailwind, and code Playground

JavaScript

var text = "DC: Batgirl, Marvel: Rogue, Image: Spawn, Image: Celestine, Marvel: Iron Man, DC: Superman, Image: Ant",
    brandRegexp = /(^|, )(\S+):/g, 
    regexp, brands, res, heroes, brand;

brands = {};
while (res = brandRegexp.exec(text)) {
    brand = res[2];
    brands[brand] ? brands[brand]++ : brands[brand] = 1;
}
for (brand in brands) { 
    regexp = new RegExp(brand + ": ([\\S ]+?)(,|$)","g");
    heroes = [];
    while (res = regexp.exec(text)) {
        heroes.push(res[1]);
    }
    console.log("Found", brands[brand], "heroes for", brand, "comics:", heroes.join(", "));
}