annyeonghaseyo

Hello, Jocar. Actually there is no specific expression like in English. The most commonly used expression when you meet someone you know is "An Yeong or An Yeong Ha Se Yo" The first one - An Yeong can be used for your friends or people younger than you. The second one -An Yeong Ha Se Yo can be used for more formal situation. You can even say to Korean President as well. But if you say to littel boys or girls, it might sound weird.