sed Command – Remove Everything After FQDN Dot

regular expressionsed

I am new to sed and am having some troubles making it work.

What I want is this:

abc.ztx.com. A 132.123.12.44 ---> abc.ztx.com

I used the below pattern, but doesn't seem to work:

echo "abc.ztx.com. A 132.123.12.44" | sed 's/\.\s.+//g'

I verified the regex using regex101.com and pattern, \.\s.+ matches the part . A 132.123.12.44 perfectly. Why is it not working with sed.

Appreciate your help. Thank you.

Best Answer

sed uses POSIX basic regular expressions (BRE) by default. \s is a PCRE (Perl-compatible regular expression) which is equivalent to the BRE [[:blank:]] (I think, matching spaces and tabs, or possiby [[:space:]] which matches a larger set of whitespace characters). The + is a POSIX extended regular expression (ERE) modifier, which is equivalent to \{1,\} as a BRE.

So try

sed 's/\.[[:blank:]].*//'

instead. You may replace [[:blank:]] by a space character if you don't need to match tabs:

sed 's/\. .*//'

Note that there is no need to do the substitution with the g flag as there will only ever be a single match. Also, the .+ that you use could just be replaced by .* instead of .\{1,\} as we don't care whether there are any further characters at all (just delete all of them).

Why does my regular expression work in X but not in Y?

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The sequence s/<[^>]*>//g is a command to the sed processing engine; it tells it to do a "Search and replace". So -e 's/..../g' means "add this search and replace command to the execution of sed.

This may make more sense if we do multiple commands in one command:

sed -e '1d' -e '$d'

would add two commands to the sed processing; "delete first line" and "delete last line".

The [^>] is a regular expression that means "any character except for the >

So [^>]* means "zero or more of any character except for the >

And so <[^>]*> means a <, optionally followed by non->, followed by >.

And then we put that into a "search and replace" command where this sequence is replaced by nothing, and then do it multiple times in the line (the final g).

This means that the string hello <abc> there <def> will first match <abc> (the <, then abc matches "zero or more non->", then the >) and replace that with nothing, and then redo this for the <def>. The result would be hello there.

(Note the extra spaces, 'cos we're not removing them!)

Best Answer

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