Often when doing a substitution for an exact word, say user to
admin, I will include spaces on either end of the regex pattern to avoid
unintentional replacements. For example, I may use something like
:%s/ user / admin /
in order to avoid a substitution like username to adminname.
In this case, the spaces can be replaced with zero-width regex characters
that match the beginning and end of a word. These are \< and \>,
respectively. Utilizing these, the previous substitution can be achieved
with
:%s/\<user\>/admin/
See :h /\< and :h /\> for more details.