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?

{
    return ((token == tokens) || ...);
}
bool ParserCastOperator::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    
    const char * data_begin = pos->begin;
    const char * data_end = pos->end;
    bool is_string_literal = pos->type == TokenType::StringLiteral;
    if (pos->type == TokenType::Minus)
    {
        ++pos;
        if (pos->type != TokenType::Number)
            return false;
        data_end = pos->end;
        ++pos;
    }
    else if (pos->type == TokenType::Number || is_string_literal)
    {
        ++pos;
    }