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?

{
            ++pos;
            if (pos < end && *pos == '=')
                return Token(TokenType::GreaterOrEquals, token_begin, ++pos);
            return Token(TokenType::Greater, token_begin, pos);
        }
        case '?':
            return Token(TokenType::QuestionMark, token_begin, ++pos);
        case ':':
        {
            ++pos;
            if (pos < end && *pos == ':')
                return Token(TokenType::DoubleColon, token_begin, ++pos);
            return Token(TokenType::Colon, token_begin, pos);
        }
        case '|':
        {
            ++pos;
            if (pos < end && *pos == '|')
                return Token(TokenType::Concatenation, token_begin, ++pos);
            return Token(TokenType::PipeMark, token_begin, pos);
        }
        case '@':
        {
            ++pos;
            if (pos < end && *pos == '@')
                return Token(TokenType::DoubleAt, token_begin, ++pos);
            return Token(TokenType::At, token_begin, pos);
        }
        case '\\':