I'm trying to learn English and I would like to know what are the expressions of surprise with positive meaning (slang or not, but not vulgar) currently used in spoken English for USA and Britain.
Is "whoa" correct? Or is "wow"?
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If you look at the context in which the words appear, for example, in the Corpus of Contemporary American English, one distinction between the words is that whoa is also used with the meaning "slow down, wait" (from its meaning as a command to a horse), whereas wow seems to be generally only used for surprise. This is even more apparent in the results in the British National Corpus. There, also, some of the results for wow are actually for wowee, a version I would have considered colloquial or outdated-sounding if hearing in AmE (but note that the BNC data also stops at the 1990s).
One discussion of wow and whoa mentions:
In the comments on that discussion, a number of speakers of different dialects offer insights. One BrE speaker mentions about spelling: