ctRegex not matching correctly at word boundaries

[Ivo]

First of all, I'm not an expert about regular expression syntax, 
however I believe there might be some strange bug when using 
ctRegex.

Consider the following code:

void main() {
         import std.regex: ctRegex, matchFirst;
	auto expression = ctRegex!("\bis\b");
	auto match = matchFirst("This island is beautiful", expression);
	if(match.empty) writeln("no match");
	else writeln("match");
}
when I compile and run I get the output "no match".

Looking online I found this website 
https://www.regular-expressions.info/wordboundaries.html where 
they explain that the above regex should match that string.

So I don't understand what's wrong with my code.

P.S.: using regex instead of ctRegex doesn't solve the problem.