There are several legitimate organizations that conduct full-contact matches. Each has its own rules that help prevent serious injury. All fighters like to fight and an injury will interfere with their being able to fight; so they do not want to be so injured in a fight that they cannot fight again later. Legitimate fighters have official records of their wins and loses, so any claims they make may be verified. Street fighter records are kept at the court house. Street fighter claims of prowess are instantiated so their claims of being a great street fighter are usually a figment of their own limited imagination. If they ever had any street fights, they were probably when they beat up drunks or some other practically defenseless person.
So called street fighters claim that ring fighters would never win in a street fight. That may be true if they were to fight by ring rules. If they fight by street fight rules, they would probably win since they are usually more disciplined and better conditioned than street fighters. Ring fighters would only have to fight in the same manner as they do in the ring, except without their normal restraints. Whereas, street fighters fighting under ring rules, would probably loose since they would have to restrain themselves.
So called street fighters like to question ring fighters with "what if" questions, such as "What if I pulled a knife?" My answer would be an "if then" statement, such as "If you pulled a knife, then I would pull my gun." I live in a concealed-carry state so I can and do legally carry a concealed firearm.