Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

if (*start_pos == 'x' || *start_pos == 'X')
        {
            ++start_pos;
            --size;
            if (parseNumber(start_pos, size, negative, 16, res))
            {
                auto literal = std::make_shared<ASTLiteral>(res);
                literal->begin = literal_begin;
                literal->end = ++pos;
                node = literal;
                return true;
            }
        }
        else
        {
            
            while (*start_pos == '0')
            {
                ++start_pos;
                --size;
            }
            if (parseNumber(start_pos, size, negative, 10, res))
            {
                auto literal = std::make_shared<ASTLiteral>(res);
                literal->begin = literal_begin;
                literal->end = ++pos;
                node = literal;
                return true;
            }