echo "'$SHELL'"       ->      outputs 'bash'
echo '"$SHELL"'       ->      outputs "$SHELL"

##</code><code>##

echo "a ' quote"' and a " quote'
outputs: a ' quote and a " quote

##</code><code>##

cat file.txt |  perl -ne '{chomp; ($a, $b) = split/\s+/; $s = "$a $b"; next if($s !~ /^\w+\'"'"'?\w+?\s+\w+\'"'"'?\w+?$/); print "$_\n"}' | wc -l