Hi monks, I only wanna to match a string that could not contain a sub-string.Such as

#!/usr/bin/perl
$s = "a000000000b";
if ($s =~ /^a.*(?!ab).*b$/) {
    print "yes";
} else {
        print "no";
}
[download]

My ideal result is to match string that : 
1)start with "a"
2)end with "b"
3)don't include "ab" between them.

That is to say,
"a000000000b" : matched
"a0a0000b00b" : matched
but
"a0000ab000b" : unmatched

I use negative look-ahead assertion (?!ab) ,but it can't work.Can somebody help me?
Thanks
Pysome