Final answer:
The prefix in 'regroup' is 're-', indicating the action of forming groups again or reorganizing. It is common in the English language and not typically associated with numerical prefixes.
Step-by-step explanation:
The prefix in the word 'regroup' is 're-'. A prefix is a group of letters added to the beginning of a word to create a new word or to change the word's meaning. The prefix 're-' generally means 'again' or 'back'. In the context of the word 'regroup', it suggests the action of forming groups again or reorganizing. In numerical prefixes, which are used to indicate multiples or fractions of units, 're-' is not typically used. However, understanding common prefixes like 're-' can be helpful in various academic subjects, including English and vocabulary building.