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?

With [true regular languages](http://en.wikipedia.org/wiki/Regular_expression#Patterns_for_non--regular_languages), technically the empty set is a [regular expression](http://en.wikipedia.org/wiki/Regular_expression#Formal_language_theory). — Oliver Charlesworth, Jan 02 '13 at 00:21
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 — mvp, Jan 02 '13 at 00:46
@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? — Mischa, Jan 02 '13 at 01:03
@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. — mvp, Jan 02 '13 at 01:15
@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. — Mischa, Jan 02 '13 at 09:59

score 14 · Accepted Answer · answered Jan 02 '13 at 00:25

14

Yes.

Here are a few examples.

.^
$.
(?!)

Naturally, there are an infinite number of such expressions.

answered Jan 02 '13 at 00:25

Kendall Frey

3

@Pterodactyl Certainly. Example: `.^`, `..^`, `...^`, etc. – Kendall Frey Oct 04 '13 at 22:26
About performance of the examples in Javascript / PHP : https://stackoverflow.com/a/70670013/7514010 – MMMahdy-PAPION Jan 11 '22 at 16:21

mvp · Answer 2 · 2013-01-02T00:33:49.193

2

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

$x^

edited Jan 02 '13 at 00:33

answered Jan 02 '13 at 00:25

mvp

I'm pretty sure it will match an empty string. – Kendall Frey Jan 02 '13 at 00:26
Well, I tried Perl flavor, and it does not match empty string – mvp Jan 02 '13 at 00:29
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/22011/discussion-between-kendall-frey-and-mvp) – Kendall Frey Jan 02 '13 at 00:29
@GillBates Wow, that is certainly strange. Which engine are you using? .NET does match, but JavaScript doesn't. – Kendall Frey Jan 02 '13 at 00:35
@Gill Bates - its about '$^' version – Gill Bates Jan 02 '13 at 00:40

score 0 · Answer 3 · edited Jan 02 '13 at 00:52

0

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

edited Jan 02 '13 at 00:52

René Höhle

answered Jan 02 '13 at 00:27

sirezekiel

3 Answers3