Phonetic fonts in lua

I am using lua making an app about phonetic alphabet learning game, I have a question that I need to check the length of the phonetic transcription and need to check is it match what I typed, but I found all phonetic alphabet's length are 2 and, and they cannot match what I typed, the example is as below:

print(string.len("ð")       -- outcome is: 2
print(string.len("pɛt"))    -- outcome is: 4
print(string.sub("pɛt",3))  -- outcome is:›t

what should I do? thanks in advance!!!!! >,<