Re is an interjection commonly used in the greek language but also used in tha Balkans. It is used in speech to gain someone's attention, add emphasis, insult, or express surprise or astonishment, like the Spanish che.
The word probably comes from μωρέ, the vocative of the ancient Greek μωρός 'dull, sluggish, foolish, stupid'.
re is often accompanied by a slang word but generally speaking you can use it any given sentence.
Re malaka how did you crash the car where you drinking and driving again?
I told you a 1000 times re I do not do this stuff anymore.
Used in emails, it is a common misconception that this is an abbreviation of the word 'regarding'.
Re is a latin word (pronounced 'ray') that means 'in the matter of' and was adopted by the English courts along with many other latin phrases. It has since moved into general correspondence.
This use to be a Greekinsult and the complete word is more(2 syllables, mo and re). Now it is commonly used amongst friends at the end of sentences or at the beginning to refer to the person that they are talking to.