Facing problem with Regex Inside Sed Command

Question

Suppose I run the following commands:

export STR="abcdef.ghijkl.mnopqr.stuvwy.log"
echo $STR | sed 's/\.[^.]*$//'

I am getting the following result:

abcdef.ghijkl.mnopqr.stuvwy

Please help me understand the above result.

Answer 1

Your sed pattern \.[^.]*$ has only one match to the original string: .log.

Details:

• \. match only a dot character.
• [^.] matches any character different from .
• [^.]* matches any sequence of characters different from .
• $ matches the end of line.

So here the final .log is the only match (.stuvwy.log is not a match because it contains an internal dot). sed will substitute this by the empty string as requested by the command s/\.[^.]*$//. Therefore you end up with:

abcdef.ghijkl.mnopqr.stuvwy

Answer 2

s/\.[^.]*$// == Substitute the first part of the string (because there's no g option at the end to match all occurrences) which starts with a dot (\.), followed by zero or more (*) characters which are not dots ([^.]), placed at the end of the string ($), with the empty string (//).

Answer 3

If you don't want to use sed you can use the shell's built-in pattern replacement. This is actually faster, not having to call an external program.

${VAR%.*} will remove the match of the glob pattern .* from the end of the string in $VAR. Remember it's a glob and not regex, so a . means a literal ".".

$ foo=aaa.bbbb.2011.log
$ echo ${foo%.*}
aaa.bbbb.2011

See Shell-Parameter-Expansion in the BASH man page for more information.