Police are searching for a robbery suspect who attacked a woman after following her from a South Boston convenience store Tuesday.The man, who was armed with a knife, followed the victim from a convenience store at K and East 7th streets at 8:30 p.m. before he approached her from behind and tried to take her bag, police said.The victim said the man pulled out the knife, punched her in the face, police said.A resident – who requested anonymity – said she and her daughter were among the people who helped the victim. “She said that a guy (had) come up behind her and put a knife to her neck and asked her for her money. He said to her, 'I know you have a lot of money. I saw you at the convenience store,’ which is a block away,” the woman said. “She was buying lottery tickets. She said he went through her bag, threw it on the ground and he said to her, 'You have no money,' so he threw her up against the wall there, and he had her by the throat, and he punched her under her left eye." The man fled the area toward H Street, officials said.Police said the suspect is believed to be a Black man, who is about 20-30 years old. He was wearing a dark colored leather jacket and dark pants. Anyone with information is asked to call detectives at 617-343-4742.