let openers = {
    '{': '}',
    '[': ']',
    '(': ')'
};

let specialOpeners = {
    ':': 'wildcard'
};

function parser(str, context = '{') {
    str = str.replaceAll(/^\s+([^\s]+)/gm,'$1')
    str = str.replaceAll(/([^\s]+)\s+$/gm,'$1')
    let nextI = str.search(/[^A-Za-z0-9\s]/);

    if (context == '[') {
        nextI = str.search(/[\]]/);
    }
    if (nextI == -1) nextI = 0;

    let nextChar = str[nextI];

    let special = specialOpeners[context] || '';

    let sp = (special == 'wildcard');

    if (nextChar == openers[context]) {
        return { context, str: str.slice(0, nextI), next: str.slice(nextI + 1), end: true};
    } else if (openers[context] || sp) {
        let args = [];
        let nextStr = str.slice(nextI + 1);

        while (nextStr && nextStr.length > 0) {
            args.push(parser(nextStr, nextChar));
            let a = args[args.length - 1];
            nextStr = a.next;
            if (a.str == '')  {
                args.pop();
            }
            if (a.end) break;
        }

        let o = { context: str.slice(0, nextI), args, next: nextStr, op: context };

        return o;
    } else {
        return { context, str: str.slice(0, nextI), next: str.slice(nextI + 1)};
    }
}

let data = parser(`
    on:start {
        echo[Hello World!]
        echo[Hello World!]
    }
`);

console.log(JSON.stringify(data, undefined, 1));