C++ code:

size_t find_start(const std::string& str, size_t pos) {
    int square_level = 1;
    int brace_level = 1;
    while (--pos) {
        switch (str[pos]) {
        case ']':
            square_level++;
            break;
        case '[':
            square_level--;
            if (square_level == 0) {
                return pos;
            }
            break;
        case '}':
            brace_level++;
            break;
        case '{':
            brace_level--;
            if (brace_level == 0) {
                return pos;
            }
            break;
        default:
            break;
        }
    }
    assert(false && "unmatched parens");
}