dictionary - A Regex that will never be matched by anything

Question

This might sound like a stupid question, but I had a long talk with some of my fellow developers and it sounded like a fun thing to think of.

So; 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.

Thus, there you have my reason for this "not a real question"...

Answer 1

Leverage negative lookahead:

>>> import re
>>> x=r'(?!x)x'
>>> r=re.compile(x)
>>> r.match('')
>>> r.match('x')
>>> r.match('y')

this RE is a contradiction in terms and therefore will never match anything.

NOTE:
In Python, re.match() implicitly adds a beginning-of-string anchor (A) to the start of the regular expression. This anchor is important for performance: without it, the entire string will be scanned. Those not using Python will want to add the anchor explicitly:

A(?!x)x