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;
    while (true)
    {
        pos = find_first_symbols<quote, '\\'>(pos, end);
        if (pos >= end)
            return Token(error_token, token_begin, end);
        if (*pos == quote)
        {
            ++pos;
            if (pos < end && *pos == quote)
            {
                ++pos;
                continue;
            }
            return Token(success_token, token_begin, pos);
        }
        if (*pos == '\\')
        {
            ++pos;
            if (pos >= end)
                return Token(error_token, token_begin, end);
            ++pos;
            continue;
        }
        UNREACHABLE();
    }
}
}
Token Lexer::nextToken()