I have a file-A containing some logging info. Suppose 202 lines: 
 <.... line 200 ....>
 <.... line 201 ....>
 <.... line 202 ....>

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These lines are the output of a periodical(ex. every 20min) grep (of a pattern xxx) on a certain live file-B If I just do every 20min something like 
    grep -A 3 -B 3 xxx  file-B >> file-A

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I could fill file-A with duplicates if following greps contain same lines (or parts) of previous greps. Every grep output is stored in a string 
    $out = qx/grep ... /     #note that $out contains new lines

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while the last line of file-A is obtained for example like: 
    $last = qx/tail -1 $fileA/

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I'd need then to keep of $out only the <NEW> part (if any) I thought that 
      if (($last =~ /\S/) && ($out =~/$last(.*)/)) {
         $out = $1
      }

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should have done the trick, but it doesn't. Any help to fix my wrong logic is welcome.

In reply to extract the tail from a string (with new lines) containing a substring by jjmoka

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