Why is Andy regretful? Why does he regret joining the gang?

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