The best way to make a regex matching exactly zero characters in Perl 6?

Question

How to explicitly define a regex (numbered or named capturing group), which always matches exactly zero characters?

> "abc" ~~ m/()abc/
===SORRY!=== Error while compiling:
Null regex not allowed

> my regex null {}
===SORRY!=== Error while compiling:
Null regex not allowed

> my regex null {<[]>}
regex null {<[]>}
> "abc" ~~ m/<null>abc/
False

Of course I can use something like the following, but I'm looking for an idiomatic way of doing it.

> my regex null {a ** 0}
regex null {a ** 0}
> "abc" ~~ m/<null>abc/
｢abc｣
 null => ｢｣

UPD: This works, but is it the best way?

> my regex null { '' }
regex null { '' }
> "abc" ~~ m/<null>abc/
｢abc｣
 null => ｢｣
> "abc" ~~ m/('') abc/
｢abc｣
 0 => ｢｣

Answer 1

The assertion that always succeeds and matches 0 characters is: <?>.

For example:

say so 'foo' ~~ / <?> 'foo' /;
# output: True

As raiph pointed out, the corresponding negative version, which always fails, is <!>. I've used this to insert error messages when part of a parse should not be reached if the input is valid. That section of the regex might read:

\d || [ { note "Error: the previous token must be followed by a number."; } <!> ]