In the assertion library, we have access to both:

infix fun <A : CharSequence> A?.shouldMatch(regex: String): A
infix fun <A : CharSequence> A?.shouldMatch(regex: Regex): A

Is there a reason for having only the negation for the String parameter?

What do you mean? There isn't any

A?.shouldNotMatch(regex: String)

?

There is

fun <A : CharSequence> A?.shouldNotMatch(regex: String): A

, but not

fun <A : CharSequence> A?.shouldNotMatch(regex: Regex): A

. On version 5.0.3 at least

Yeah, we forgot it. Could you you create a ticket with this information, please? 😄