Replace the "exact"
b with a character class
[bc]
Then Perl can't rule out the string because of the missing end and you'll see exponential growth.
use strict;
use warnings;
$\="\n";
$|=1;
redos($_) for 5..8;
sub redos {
my ($length)=@_;
my $futile = 'a' x $length;
print "=== length=$length string=$futile";
print 'start ', my $start = scalar time;
die 'huh?' if $futile =~
/a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a
+*a*a*[bc]/
;
print 'post rx 1 ', time -$start," sec";
die 'huh?' if $futile =~
/(?:(?:(?:(?:(?:(?:(?:(?:(?:(?:(?:(?:(?:a*)*)*)*)*)*)*)*)*)*)*)*
+)*)*[bc]/
;
print 'post rx 2 ', time -$start," sec";
print 'done ', time -$start," sec";
print "\n" x2;
}
C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/redos.pl
=== length=5 string=aaaaa
start 1633643412
post rx 1 0 sec
post rx 2 0 sec
done 0 sec
=== length=6 string=aaaaaa
start 1633643412
post rx 1 1 sec
post rx 2 1 sec
done 1 sec
=== length=7 string=aaaaaaa
start 1633643413
post rx 1 4 sec
post rx 2 4 sec
done 4 sec
=== length=8 string=aaaaaaaa
start 1633643417
post rx 1 20 sec
post rx 2 20 sec
done 20 sec
it's the first regex which is obviously growing in an exponential manner...