\B
Matches everywhere but on a word boundary; that is it matches if the character to the left and the character to the right are either both “word” characters or both “non-word” characters.
実際のところ
$ echo 完全に理解した | gsed 's/\B/ /g' 完 全 に 理 解 し た
$ echo "abc %-= def." | gsed 's/\B/X/g' aXbXc X%X-X=X dXeXf.X
