if (my ($match) = $s =~ /yesterday (.*?) after/s) {
   say $match;
}

##</code><code>##

yesterday foo yesterday bar after

##</code><code>##

foo yesterday bar

##</code><code>##

bar

##</code><code>##

if (my ($match) = $s =~ /hello ((?:(?!yesterday).)*) yesterday/s) {
   say $match;
}