Could someone explain to me why I should use `"(?<=\\s|^)@(\\w*[A-Za-z&_-]+\\w*)"` instead of just simply using `"(@\\w+)"`?

Could someone explain to me why I should use "(?<=\\s|^)@(\\w*[A-Za-z&_-]+\\w*)" instead of just simply using "(@\\w+)"?