'a(b(c)<3>de)<2>f' => 'abcccdebcccdef'

function parseStr(str){
    let execResult = /\((\w+)\)<(\d+)>/g.exec(str);
    while(execResult) {
        str = str.replace(execResult[0], execResult[1].repeat(execResult[2]));
        execResult = /\((\w+)\)<(\d+)>/g.exec(str);
    }
    return str;
}

console.log(parseStr('a(b)<10>c')); // abbbbbbbbbbc
console.log(parseStr('a(b)<2>(c)<3>')); // abbccc
console.log(parseStr('a(b(c)<3>de)<2>f')); // abcccdebcccdef