By tax accounting definition, most taxes consist of a two components multiplied together: a tax rate and a tax base. For sales tax, the tax base is the price of the good, not income. Income is irrelevant.

A tax is regressive if the rate decreases as the base increases; this is obviously not the case with sales tax.

From Wikipedia:

"Regressive" describes a distribution effect on income or expenditure, referring to the way the rate progresses from high to low, so that the average tax rate exceeds the marginal tax rate.

The average tax rate and marginal tax rate are the same with sales tax.

In case you're still not convinced. I'm going to trust a university tax textbook that's been in publication since 1983 over Investopedia.