On Mac OS X, neither of these regex syntaxes work inside sed for matching whole words
\bmyWord\b
\<myWord\>
Hear me now and believe me later, this ugly syntax is what you need to use:
/[[:<:]]myWord[[:>:]]/
So, for example, to replace mint with minty for whole words only:
sed "s/[[:<:]]mint[[:>:]]/minty/g"
Source: re_format man page