>perl -wMstrict -le
"use Regexp::Assemble;
 ;;
 my %subs = (Xaa1 => 'foo',  Xaa11 => 'bar',  Xaa => 'baz',);
 ;;
 my $ra = Regexp::Assemble
        ->new
        ->add(map quotemeta, keys %subs)
        ->anchor_word
        ;
 print $ra->re;
 ;;
 my $s = 'Xaa Xaa1 Xaa11 Xa';
 $s =~ s{ ($ra) }{$subs{$1}}xmsg;
 print qq{'$s'};
"
(?-xism:\bXaa(?:1?1)?\b)
'baz foo bar Xa'