Match sequence of zero or more characters except a given character

This looks like it should be simple: I want to match an "i" followed by an "o" with an arbitrary (possibly zero-length) sequence of characters in between, unless that sequence also contains an "i". Without this constraint it's easy: ___~~"i"~~___~~"o"~~___. This will match "histology", but it will also match "adventitious". I've tried using a conditional or a predicate test in the pattern but I can't get the syntax right.