Update: All '*' corrected to be '?' per follow ups.

but how to match s only if e is there, and match e only if s is there ?

I don't think anyone actually answered your question.

For proper abbreviations of 'yes', you can use m[^Y(?:E(?:S)?)?$]i.

That's a bit unweildy, but the repetitious nature means it is quite easy to generate a regex for any given word:

sub genAbbrevRegex{
    my( $first, @rest ) = split '', shift;
    my $re = '';
    $re = "(?:$_$re)?" for reverse @rest;
    return qr[^$first$re$]i;
}

print genAbbrevRegex( 'fred' );;
(?i-xsm:^f(?:r(?:e(?:d)?)?)?$)

$r = genAbbrevRegex( 'Yes' );;

[0] Perl> print "$_: ", m[$r]? 'ok' : 'not ok' for qw[ YES YE Y yes ye
+ y YS ES S ys es s yees yess ];;
YES:  ok
YE:  ok
Y:  ok
yes:  ok
ye:  ok
y:  ok
YS:  not ok
ES:  not ok
S:  not ok
ys:  not ok
es:  not ok
s:  not ok
yees:  not ok
yess:  not ok
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