Your question asks, “Why is Andy regretful? Why does he regret joining the gang?” I must assume that you refer to the short story, “On the Sidewalk, Bleeding” by Evan Hunter. In this short story, Andy regrets his decision to become a Royal (the name of his gang and rival to the Guardians) when he realizes that he is dying from a stab wound.
Andy is stabbed as he leaves a party (a joint) to buy cigarettes. The candy store, where he expects to make the purchase, is closed. Therefore, he decides to go to another store which is a short distance away. Unfortunately, as he enters the alley, someone stabs him and remarks, “That's for you Royal!”
Initially, despite the profusely bleeding wound, Andy does not realize that he is dying. However, once it occurs to him that no help is on the way and that he will likely die in the alley, he begins to regret his decision to join the gang. He realizes that he loves Laura, his girlfriend, and that he would like to build a life with her. He also begins to understand that his attackers did not target him personally. They simply responded to the purple silk jacket that identified him as a Royal, an enemy. Andy desperately wants to discard the jacket so that, when his body is discovered, he is identified as an individual and not simply as a gang member.
Andy realizes too late that his decision to join the Royals was a mistake. He loses his opportunity to build a better life for himself with one who loves him (Laura). He also knows that the police will see only the purple silk jacket and assume that his death was inevitable and in consequence of the gang rivalry between the Royals and the Guardians. a