my $string="123 abcdef";
$string=~s{(\d+)|(\w+)|(\s+)}
          {
          	defined($1) ? '\\d{'.length($1).'}'
                       : defined($2) ? '\\w{'.length($2).'}'
                                     : '\\s{'.length($3).'}'
          }ge;
print $string;
__END__
\d{3}\s{1}\w{6}