A prefix is a half word (e.g., anti- , ex- , post- , pre-) placed before a word to modify its meaning. Most prefixed words can be written with or without a hyphen after the prefix.|
As a guideline, avoid the hyphen if you can, but if the unhyphenated version is a spelling mistake (let your spellchecker do its job) or looks too unwieldy for you, use a hyphen.
That said, you should always use a hyphen with ex- and self-, and you should never allow ambiguity to creep in (e.g., you must write re-press the shirt not repress the shirt). There are more-detailed guidelines below.
When Do You Need a Hyphen with a Prefix?There is often confusion over whether a hyphen should be used with a prefix. In other words, should you write re-consider or reconsider, or anti-aircraft or antiaircraft?
Unfortunately, there is no simple rule governing this, but there are some guidelines.
Guiding Principles for Hyphens with PrefixesIf it's not a spelling mistake to avoid the hyphen and you can bear how the words looks without it, then avoid the hyphen.
Often, it's your choice whether to use a hyphen. Lots of prefixed words can be written with or without a hyphen. The underlying guideline is:
Use a Hyphen with a Proper NounIf your prefix sits before a proper noun, you should use a hyphen. Examples:
Do Not Allow the Same Vowel to Double UpIf the prefix ends in the same vowel that the root words starts with, separate them with a hyphen. Examples:
You Can Let Different Vowels Double UpWhen a prefix ends in a vowel and the root word starts with a different vowel, it is usual to omit the hyphen. Examples:
Use a Hyphen with Ex and SelfThe prefixes ex and self are usually followed by a hyphen. Examples:
Eliminate Ambiguity Every TimeIf the unhyphenated version could be confused with a different word, add the hyphen. (This is most common with the prefix re. Examples:
Enjoy the LeniencyMost prefixed words exist in both forms. As you might have noticed in the guidelines above, it is often down to how the writer feels about the word.
USE YOUR SPELLCHECKER AND YOUR INSTINCT
Most prefixed words exist in both forms. If you can, avoid the hyphen, but if the unhyphenated version is picked up by your spellchecker or looks too unwieldy for you, use a hyphen. The exceptions to this are:
SOME COMMON PREFIXES
Some common prefixes are: