How do you search for strings within a string?

I'm writing a pig latin decoder. This section works with 'qu' works but currently only is the third letter of the word is a vowel. I am implementing an if statement to get it to work for words that have a consonant as the third letter, but keep getting this error: TypeError: 'in ' requires string as left operand, not list

Here is my code:

if w[-2:] == 'ay':
        RegW = []
        y = w.find('-')
        beginningw = w[y:]
        if vowel not in beginningw[0]:
            RegW.append(beginningw[0:-2] + w[0:y])
        else:
            RegW.append('qu' + w[0:y])
        return RegW[0]

It works for these word: ay-quay (quay) iz-quay (quiz) eue-quay (queue)

but NOT an-quray (quran) (returns quan w/o if statement I'm trying to)