Are sales taxes regressive?
It depends on the definition you want to use. You can define a regressive tax as the wiki says, in which case a sale tax would not match. 

But the goal of definition of different class of taxation is to assess the impact on person being taxed. Without considering income the above definition becomes moot. Sure the tax is proportional (fixed share of tax on any price) but it's effect on those who pay it is different.

The first definition is derived , I believe, in the context of income taxes in which the income is being taxed and the definition matches it's intuitive notion. 
Some taxation system are subject to the requirement that the system as a whole should be progressive, I.e the total tax burden should increase with income. In this context is natural to think that sale taxes are regressive, while they don't match the formal definition.