Police are searching for a man who allegedly pepper-sprayed a woman, punched her in the face, and then stole her purse as she walked home in Charlestown Saturday night. The 28-year-old woman was found on Austin Street by police around 10:30 p.m. She told them she had been walking home from the Community College MBTA station when the man approached. After he punched her and knocked her to the ground, the victim said, he drove off in a gray minivan up School Street toward Main Street. The victim described her attacker as white, 6 feet tall, with a medium build, and wearing a black jacket. Officers searched the area but did not find the suspect. The woman was treated at the scene for her injuries. Boston police detectives asked anyone with information to call 617-343-4248.