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[0] == '/' && pos[1] == '*')
                        {
                            pos += 2;
                            ++nesting_level;
                        }
                        else if (pos[0] == '*' && pos[1] == '/')
                        {
                            pos += 2;
                            --nesting_level;
                            if (nesting_level == 0)
                                return Token(TokenType::Comment, token_begin, pos);
                        }
                        else
                            ++pos;
                    }
                    pos = end;
                    return Token(TokenType::ErrorMultilineCommentIsNotClosed, token_begin, pos);
                }
            }
            return Token(TokenType::Slash, token_begin, pos);
        }
        case '#':   
        {           
                    
                    
                    
                    
                    
            ++pos;