Regana Tosh is a Markalian smuggler whose dealings with Quark attract Worf's attention, leading Worf to arrest him and unknowingly wreck an undercover operation that Odo had been running for weeks.
Regana Tosh features in the fourth-season episode Hippocratic Oath, in the story running alongside Bashir and O'Brien's capture by the Jem'Hadar. He is a Markalian with a criminal reputation who turns up on Deep Space 9 and starts doing business with Quark.
Worf, newly arrived and still thinking like a starship security chief, cannot understand why Odo appears to be ignoring him. He follows the pair on his own initiative and arrests Tosh as he hands Quark a consignment of illegal Tallonian crystals.
Odo then explains that Quark was working for him and that Tosh was only the bait: the plan had been to follow the crystals to the smuggling ring behind him. The incident teaches Worf that policing the station is not like policing the Enterprise.