An exercise in parsing

timerparser.js

// input format
let input = '2x(1m:5s) 2x(3x(2m:1s) 3m:2s 4x(2m:2s)) 20s 3m 3s';

// recursive splitter/parser
let splitter = s=>s.split('').reduce((parse, char, idx, arr)=>{
    if (parse.depth>0){
        if (char==')'){
            parse.depth--;
            if (parse.depth)
                parse.group+=char;
            else{
                let inner = splitter(parse.group).groups;
                for(;parse.grouptimes--;parse.grouptimes>0)
                    parse.groups = parse.groups.concat(inner);
                parse.group='';
                parse.grouptimes=0;
            }
        }
        else {
            parse.group+=char;
            if (char=='(')
                parse.depth++;
        }
    }
    else if (char=='('){
        parse.depth++;
        parse.group = '';
        parse.grouptimes = parseInt(parse.accum);
        parse.accum = '';
    }
    else if (char==')')
        throw 'unbalanced parentheses';
    else {
        if (char!=' ')
            parse.accum+=char;
        if (char==' ' || idx==arr.length-1){
            if (parse.accum.length){
                let group = /^(?:(\d+m):?)?(\d+s)?$/i.exec(parse.accum);
                parse.groups.push({min: parseInt(group[1]||0), sec: parseInt(group[2]||0)});
                parse.accum = '';
            }
        }
    }
    return parse;
}, {groups: [], accum: '', group: '', grouptimes: 0, depth: 0});

// test
console.log(splitter(input).groups.map(entry=>`${entry.min}m:${entry.sec}s`).join(' '))
// should return
// 1m:5s 1m:5s 2m:1s 2m:1s 2m:1s 3m:2s 2m:2s 2m:2s 2m:2s 2m:2s 2m:1s 2m:1s 2m:1s 3m:2s 2m:2s 2m:2s 2m:2s 2m:2s 0m:20s 3m:0s 0m:3s