sed: How to delete lines matching a pattern that contains forward slashes?

Question

Suppose a file /etc/fstab contains the following:

/dev/xvda1 / ext4 defaults 1 1
/dev/md0    /mnt/ibsraid    xfs defaults,noatime    0   2
/mnt/ibsraid/varlog /var/log    none    bind    0   0
/dev/xvdb   None    auto    defaults,nobootwait 0   2

I want to delete the line starting with /dev/xvdb. So I tried:

$ sed '/^/dev/xvdb/d' /etc/fstab
sed: -e expression #1, char 5: extra characters after command
$ sed '?^/dev/xvdb?d' /etc/fstab
sed: -e expression #1, char 1: unknown command: `?'
$ sed '|^/dev/xvdb|d' /etc/fstab
sed: -e expression #1, char 1: unknown command: `|'

None of these worked. I tried changing the delimiters to ? and | because doing this works for the sed substitution command when a pattern contains /.

I am using GNU Sed 4.2.1 on Debian.

Answer 1

You were very close. When you use a nonstandard character for a pattern delimiter, such as |pattern|, the first use of that character must be escaped:

$ sed '\|^/dev/xvdb|d' /etc/fstab /dev/xvda1 / ext4 defaults 1 1 /dev/md0    /mnt/ibsraid    xfs defaults,noatime    0   2 /mnt/ibsraid/varlog /var/log    none    bind    0   0 

Similarly, one can use:

sed '\?^/dev/xvdb?d' /etc/fstab 

Lastly, it is possible to use slashes inside of /pattern/ if they are escaped in the way that you showed in your answer.

Answer 2

After some digging, I found that it is possible to escape the / in the pattern string using \. So this works:

$ sed '/^\/dev\/xvdb/d' /etc/fstab