Get the contents between the 2 given strings : 
How do I get the contents between the 2 given strings?
There is no output.

     keyword_01=’Serial\s\+Number’;
     keyword_02='Signature\s\+Algorithm';

     openssl s_client -servername example.com -connect example.com:443 </dev/null 2>/dev/null | openssl x509 -text | grep -i -o -P --color=auto '(?<=$keyword_01).*(?=$keyword_02)'

The basic recommendations work, but when it is pieced as above, it doesn't.

     echo "Here is a string" | grep -o -P --color=auto '(?<=Here).*(?=string)'
