What is the ruling on the widespread practice among youth of "al-alsh" (playful teasing/banter), which includes using phrases such as "Once I went to befriend someone, and he turned out to be a zero," or "a blanket," with the response being, "nor a cooked duck?"

The kind of joking mentioned in the question is permissible as long as it is minimal and does not involve falsehood or lies.

Regarding the first phrase: If the speaker intends that the person is of no benefit or does not help anyone, then there is no harm in that. Otherwise, it is a lie, and lying in jokes is not permissible, due to the Prophet's (peace and blessings be upon him) saying: "Woe to him who speaks and lies to make people laugh thereby. Woe to him! Woe to him!"

As for the second phrase, it does not appear to fall under forbidden joking.