What's your thought - what does a Regex look like, that will never be matched by any string, ever!
Edit: Why I want this? Well, firstly because I find it interesting to think of such an expression and secondly because I need it for a script.
In that script I define a dictionary as Dictionary<string, Regex>. This contains, as you see, a string and an expression.
Based on that dictionary I create methods that all use this dictionary as only reference on how they should do their work, one of them matches the regexes against a parsed logfile.
If an expression is matched, another Dictionary<string, long> is added a value that is returned by the expression. So, to catch any log-messages that are not matched by an expression in the dictionary I created a new group called "unknown".
To this group everything that didn't match anything other is added. But to prevent the "unknown"-expression to mismatch (by accident) a log-message, I had to create an expression that is most certainly never matched, no matter what string I give it.
This is actually quite simple, although it depends on the implementation / flags*:
$a
Will match a character a after the end of the string. Good luck.
WARNING:
This expression is expensive -- it will scan the entire line, find the end-of-line anchor, and only then not find the a and return a negative match. (See comment below for more detail.)
* Originally I did not give much thought on multiline-mode regexp, where $ also matches the end of a line. In fact, it would match the empty string right before the newline, so an ordinary character like a can never appear after $.