How to set up REGEX that doesn't match anything?

Question

Just for curiosity.

Is it possible to create a regular expression that will not match any string, including an empty string?

Apeldoorn

With true regular languages, technically the empty set is a regular expression. – Antiperiodic

I guess it is the same point as having NULL, undef or None: sometimes you want to have compiled regex as starting value that never matches anything – Nils

@Nils Exactly! (I wrote this to avoid limitation) – Apeldoorn

@GillBates, I removed your remark about the closure from the question. It should be a comment. You may be right that it shouldn't have been closed, but you got your answer, so it doesn't really matter, does it? – Rigdon

@Mischa: this is rather strange attitude. If you think that it should not have been closed, question is clear, good answer exists, why you close it as not a real question? If may not matter for this user right now, but it WILL matter to other users who will read this question later. – Nils

@mvp, once you vote to close, you cannot undo it. I had already voted to close, but later when I read all comments and answers I thought I shouldn't have. – Rigdon

Answer 1

Yes.

Here are a few examples.

.^
$.
(?!)

Naturally, there are an infinite number of such expressions.

Ida

@Pterodactyl Certainly. Example: .^, ..^, ...^, etc. – Ida

About performance of the examples in Javascript / PHP : https://mcmap.net/q/25797/-a-regex-that-will-never-be-matched-by-anything – Fox

Answer 2

This regex should never match anything (provided you do not use single-line or multi-line modifiers):

$x^

Nils

I'm pretty sure it will match an empty string. – Ida

Well, I tried Perl flavor, and it does not match empty string – Nils

let us continue this discussion in chat – Ida

@GillBates Wow, that is certainly strange. Which engine are you using? .NET does match, but JavaScript doesn't. – Ida

@Gill Bates - its about '$^' version – Apeldoorn

Answer 3

How about /^$x/. When I try it with ruby, it seems to work.

Multiangular