Is there a Regular Expression that will never matc

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Closed 7 years ago.

Sort of a two part question:

1. Is there any theoretical regular expression that will never match any string (using general syntax without any fancy stuff provided by modern regular expression matchers)?
2. Is there a simple way to use C#'s Regex syntax to create a regex that will never match any string (this time, all the fancy stuff is included)?

NOTE: I am not referring to matching the empty string (that would be easy, just "").

Answer 1

Without multi-line mode, the end doesn't usually tend to appear before the beginning:

$.^

Or more simply, again without multi-line mode:

$.

With lookarounds, you can do all kinds of contradictory stuff:

(?=a)(?=b)

This forces a character to be two different things at once, which is of course impossible.

Answer 2

Just as you can match any characters with [\s\S], you can match no characters with [^\s\S] (or [^\w\W], etc).

Answer 3

You could use contradictory lookbehinds, for example

\w(?<!\w)

Here \w will match any word character and the lookbehind (?<!\w) will make sure that the last character was not a word.