from Latin re-, red- 'again, back'.
In modern English the tendency is for words formed with prefixes such as re- to be unhyphenated: restore, remain, reacquaint. One general exception to this is when the word to which re- attaches begins with e: in this case a hyphen is often inserted for clarity: re-examine, re-enter, re-enact. A hyphen is sometimes also used where the word formed with the prefix would be identical to an already existing word: re-cover (meaning ‘cover again’, as in we decided to re-cover the dining-room chairs) not recover (meaning ‘get better in health’). Similar guidelines apply to other prefixes, such as pre-.