Question

Is is possible to construct a regular expression that rejects all input strings?

Answer 1

Probably this:

[^\w\W] 

\w – word character (letter, digit, etc)
\W – opposite of \w

[^\w\W] – should always fail, because any character should belong to one of the character classes – \w or \W

Another snippets:

$.^ 

$ – assert position at the end of the string
^ – assert position at the start of the line
. – any char

(?#it's just a comment inside of empty regex) 

Empty lookahead/behind should work:

(?<!)