68 characters
/\w\b\w/
A word boundary (\b) surrounded by 'word' characters (\w - one of [_a-zA-Z0-9]). It is unmatchable since one of the characters preceding or following a word boundary must be a non-'word' character.
By the way: this is similar to the unmatchable expression
/\W\b\W/
where \W means non-'word' character.