Hide part of match from grep output

Question

I have a regex for session ID extraction:

[root@docker tmp]# grep -oE "\[[0-9].+\]" logfile
[113a6d9e-7b06-42c6-a52b-7a4e4d2e216c]
[113a6d9e-7b06-42c6-a52b-7a4e4d2e216c]
[root@docker tmp]#

How can I hide the square brackets from the output?

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

Answer 1

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

$ grep -oP "(?<=\[)[0-9].+(?=\])" logfile
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c

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.

Answer 2

$ 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
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c

https://stackoverflow.com/a/19242713/6947646

Answer 3

sed is more applicable for the case than grep

sed '/\n/{P;D;};s/\[/\n/;s/\]/\n/;D' log

• Raja G: Hey, could you please explain what sed doing here. Thank you.

Answer 4

One way is piping to cut:

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

Answer 5

The simplest way would be to let tr delete them:

$ grep -oE "\[[0-9].+\]" logfile | tr -d '[]'
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c
113a6d9e-7b06-42c6-a52b-7a4e4d2e216c

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 ].

• Raja G: Yes , i tried that as well and same answer already posted as comment by @Costas