A prefix is a small word element that is attached to a following word, for example in sub-Saharan and submarine, where sub- means ‘below’ or ‘under’. A hyphen is used with some prefixes in some cases.

The table below lists a number of prefixes common in university and research writing:

Prefix Without Hyphen With Hyphen
anti antibiotic anti-American
co cooperative co-operative
intra intravenous intra-arterial
micro microbiology micro-organism
neo neoclassical neo-Nazism
non nonconformist non-nuclear
post postwar post-Keynesian
pre prerequisite Pre-Raphaelite
pro proceed pro-American
re reform re-form
self selfish self-absorbed
semi semicircle semi-independent
sub submarine sub-Saharan

Most prefixes do not normally use a hyphen; however, they may do so in the cases discussed below.

When to use a hyphen with a prefix

To make a word easier to read

A hyphen can be used to make a word easier to read:

Where the prefix and root have the same letter

meta-analysis
pre-emptive
re-entry
anti-inflammatory
non-nuclear
co-occur
cross-sectional

But

override
overrated
underrated
underrepresented

To avoid a confusing combination of letters

anti-apartheid

Before a capital letter

non-Hodgkin’s lymphoma
pan-African
post-Keynesian
Pre-Raphaelite
Proto-Indo-European
sub-Saharan

With an abbreviation

non-SI
pro-EU
anti-US

With a date

pre-1900
post-1945

To avoid ambiguity

to reform the United Nations
to re-form the United Nations

With self-

Most compounds with self- are written with a hyphen:

self-adhesive
self-confidence
self-rule

But

selfish
selfless

non-

Non is somewhat of an exception, as both forms with and without a hyphen are found:

Without Hyphen With Hyphen
nonessential non-essential
nonfiction non-fiction
nonproliferation non-proliferation

US English tends to use non without a hyphen (apart from cases such as non-nuclear where the following word begins with n). In British English, both styles are used.

In your text, you should decide whether to use non with or without a hyphen, following the style of your target publication or a single dictionary as your guide.

Note: your word processor may not distinguish systematically between non spellings with and without a hyphen.