1 attachable to almost any verb or its derivative, meaning: a once more; afresh, anew (readjust; renumber). b back; with return to a previous state (reassemble; reverse).
A hyphen is normally used when the word begins with e (re-enact), or to distinguish the compound from a more familiar one-word form (re-form = form again).
2 (also red- before a vowel, as in redolent) in verbs and verbal derivatives denoting: a in return; mutually (react; resemble). b opposition (repel; resist). c behind or after (relic; remain). d retirement or secrecy (recluse; reticence). e off, away, down (recede; relegate; repress). f frequentative or intensive force (redouble; refine; resplendent). g negative force (recant; reveal).
L re-, red-, again, back, etc.