Even though it might be considered a really, really emphatic variation of 'Yes', the solution of Re: conditional regex will accept something like 'Yeeessseeseeesssssess'. Wouldn't an approach like the following be better:

>perl -wMstrict -le
"sub genAbbrevRegex{
     my ($first, @rest) = split '', shift;
     my $re = '';
     $re = qq{(?: $_ $re)?} for reverse @rest;
     return qr{ $first $re }xmsi;
     }
 print genAbbrevRegex( 'fred' );;
 my $r = genAbbrevRegex( 'Yes' );;
 print $r;
 print qq{'$_': }, m{ \A $r \z }xms ? 'ok' : 'not ok'
   for qw[ YES YE Y yes ye y ys es s e Yeeessseeseeesssssess];;
"
(?msix: f (?: r (?: e (?: d )?)?)? )
(?msix: Y (?: e (?: s )?)? )
'YES': ok
'YE': ok
'Y': ok
'yes': ok
'ye': ok
'y': ok
'ys': not ok
'es': not ok
's': not ok
'e': not ok
'Yeeessseeseeesssssess': not ok
[download]

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