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