Desktop to IOS

I am trying to take some code from desktop to IOS. Apart from my confusion with String and Text and some tings not giving me autocomplete options, what is my equivalent for converting

vowelpos = instr(1,"AEIOU",uppercase(letter(w,z)))

where letter was a String array hold letters

var base as string=“AEIOU”
vowelpos=base.indexOf(0, letter(w, z).Uppercase), ComparisonOptions.CaseSensitive)

Thanks Antonio