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 isAlphaASCII(c)
        || isNumericASCII(c);
}
inline bool isWordCharASCII(char c)
{
    return isAlphaNumericASCII(c)
        || c == '_';
}
inline bool isValidIdentifierBegin(char c)
{
    return isAlphaASCII(c)
        || c == '_';
}
inline bool isWhitespaceASCII(char c)
{
    return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f' || c == '\v';
}
inline bool isWhitespaceASCIIOneLine(char c)
{
    return c == ' ' || c == '\t' || c == '\f' || c == '\v';
}
inline bool isControlASCII(char c)
{
    return static_cast<unsigned char>(c) <= 31;
}
inline bool isPrintableASCII(char c)