Can't "select all" to delete when searching emails - ios7
I have just done a search of my yahoo inbox and I want to select all of the results that it came up with so I can delete them all.
There is an edit button in the bottom left corner, but when I press it the only option I have is to individually select them all.
Does anyone know another way I can go about doing this - I want to search for emails from certain senders, subjects, websites, etc; I have over 34000 emails many do which need to go, but there are some really important ones in there too... somewhere!
iPhone 5, iOS 7