不是很清楚你的具體需求，直接提取不就是了

const str = "......";

const result = str.match(/!\[(.*?)\]\((.*?)\)/);

console.log(result); // ["", "a", "b"]

獲取多個的話可以用exec

const str = ".........";

const pattern = /!\[(.*?)\]\((.*?)\)/mg;

const result = [];

let matcher;

while ((matcher = pattern.exec(str)) !== null) {

    result.push({

        alt: matcher[1],

        url: matcher[2]

    });

}

console.log(result); // [{ alt: 'a', url: 'b' }, { alt: 'c', url: 'd' }]