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 (pos->type != TokenType::OpeningRoundBracket)
        return false;
    ++pos;
    ParserList columns_p(std::make_unique<ParserCompoundIdentifier>(false, true), std::make_unique<ParserToken>(TokenType::Comma), false);
    ParserStringLiteral regex;
    ASTPtr column_list;
    ASTPtr regex_node;
    if (!columns_p.parse(pos, column_list, expected) && !regex.parse(pos, regex_node, expected))
        return false;
    if (pos->type != TokenType::ClosingRoundBracket)
        return false;
    ++pos;