Grouping can be used in sed like normal regular expression. A group is opened with “\(” and closed with “\)”.Grouping can be used in combination with back-referencing.
Back-reference is the re-use of a part of a Regular Expression selected by grouping. Back-references in sed can be used in both a Regular Expression and in the replacement part of the substitute command.
Example 1: Get only the first path in each line
$ sed 's/\(\/[^:]*\).*/\1/g' path.txt
/usr/kbos/bin
/usr/local/sbin
/opt/omni/lbin
In the above example, \(\/[^:]*\) matches the path available before first : comes. \1 replaces the first matched group.
Example 2: Multigrouping
In the file path.txt change the order of field in the last line of the file.
$ sed '$s@\([^:]*\):\([^:]*\):\([^:]*\)@\3:\2:\1@g' path.txt
/usr/kbos/bin:/usr/local/bin:/usr/jbin:/usr/bin:/usr/sas/bin
/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/opt/omni/bin:
/root/bin:/opt/omni/sbin:/opt/omni/lbin
In the above command $ specifies substitution to happen only for the last line.Output shows that the order of the path values in the last line has been reversed.
Example 3: Get the list of usernames in /etc/passwd file
This sed example displays only the first field from the /etc/passwd file.
$sed 's/\([^:]*\).*/\1/' /etc/passwd
root
bin
daemon
adm
lp
sync
shutdown
Example 4: Parenthesize first character of each word
This sed example prints the first character of every word in paranthesis.
$ echo "Welcome To The Geek Stuff" | sed 's/\(\b[A-Z]\)/\(\1\)/g'
(W)elcome (T)o (T)he (G)eek (S)tuff
Example 5: Commify the simple number.
Let us create file called numbers which has list of numbers. The below sed command example is used to commify the numbers till thousands.
$ cat numbers
1234
12121
3434
123
$sed 's/\(^\|[^0-9.]\)\([0-9]\+\)\([0-9]\{3\}\)/\1\2,\3/g' numbers
1,234
12,121
3,434
123
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