You are apparently looking for word search with case-sensitivity.
For case-sensitive search, set
You can also just toggle case-sensitivity temporarily, during Isearch, using
For incremental word search, use
M-s w. For nonincremental word search, use
M-s w RET.
There are additional ways to do word search. See the Emacs manual, node Word Search.
Strike that, for you apparently do not want to find
abcDEF when it follows
.. So you want something different from word search.
In that case, use regexp search, looking for whitespace followed by word-constituent characters, e.g.
That includes the whitespace in the search hit. If you want to be able to see the word itself highlighted differently from the total match, use library
isearch+.el and search with this:
C-M-s [[:space:]]\(abcDEF\). Isearch+ highlights each regexp group (e.g.