I have a regex for session ID extraction:

[root@docker tmp]# grep -oE "\[[0-9].+\]" logfile
[root@docker tmp]#

How can I hide the square brackets from the output?

  • can you post the contents of your logfile. may be you need awk command to process and get the desired output – Kamaraj Feb 28 '17 at 9:32
  • @Kamaraj , Thank you for responding. awk is in my mind. But I am trying to solve this with only grep+regex. – rɑːdʒɑ Feb 28 '17 at 9:33
  • can you post the contents of log file.. atleast 2 lines – Kamaraj Feb 28 '17 at 9:34
  • 1
    Second you can pipe output trough | tr -d '][' e.g. – Costas Feb 28 '17 at 9:35
  • maybe this will work : grep -oE "[[0-9].+]" logifle | awk -F"[][]" '{print $2}' – Rakesh.N Feb 28 '17 at 9:36

Instead of using extended-regex grep (-E), use perl-regex grep instead (-P), with a lookbehind and lookahead.

$ grep -oP "(?<=\[)[0-9].+(?=\])" logfile

Here, (?<=\[) indicates that there should be a preceding \[, and (?=\]) indicates that there should be a following \], but not to include them in the match output.

$ cat a.txt
test hello..[113a6d9e-7b06-42c6-a52b-7a4e4d2e216c]... this is
te [113a6d9e-7b06-42c6-a52b-7a4e4d2e216c].  this  is hello

$ grep -oP '(?<=\[)[^\]]*' a.txt



One way is piping to cut:

grep -oE "\[[0-9].+\]" logfile | cut -d'[' -f2 | cut -d']' -f1

The simplest way would be to let tr delete them:

$ grep -oE "\[[0-9].+\]" logfile | tr -d '[]'

Notice that the tr utility doesn't know about regular expressions or patterns in quite the same way as the shell does. In this case, the [] operand is just the two characters [ and ].

  • Yes , i tried that as well and same answer already posted as comment by @Costas – rɑːdʒɑ Feb 28 '17 at 9:45

sed is more applicable for the case than grep

sed '/\n/{P;D;};s/\[/\n/;s/\]/\n/;D' log
  • Hey, could you please explain what sed doing here. Thank you. – rɑːdʒɑ Sep 22 '18 at 2:09

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