Hi,

can some one suggest me,how "sed" is managed to delete the second field here.

Any explanation on , how the below code is working would be appreciated.

sed 's/^\([^:]*\):[^:]:/\1::/'  /etc/passwd

sed 's/[^:]*:/:/2'  /etc/passwd

Hi, the first one deletes the second field only if it consists of one character... It replace the first field and a colon + a character and a colon with the first field (that was captured with \( .. \) and recalled by \1 ) and two colons

The second replaces any number of non-colons and a colon by a colon, the number two means the it does this with the second occurrence on a line...

Hi Scrutinizer,

Thanks for the reply. I got your second correctly.

Your first answer is slightly confusing me!..

Let's say the sample input is:

abcd:x:panyam:panyam:Panyam:512

echo "abcd:x:panyam:panyam:Panyam:512" | sed 's/^\([^:]*\):[^:]:/\1::/' gives me:

abcd::panyam:panyam:Panyam:512

"panyam:panyam:Panyam:512"

Hi Panyam, yes the rest of the line remains unaltered. It is not part of the substitution, so it gets printed like it is.

An attempt to further explain the regular expression, with an ulterior motive of setting up for a question.

Given:

sed 's/^\([^:]*\):[^:]:/\1::/'

^    = start of the line
\(   = Start of first remembered pattern
[^:] = followed by any character that is not a :
*    = followed by any number of the previous character class
       (characters that are not colons)
\)   = end of first remembered pattern
:    = followed by a colon
[^:] = followed by any character that is not a colon
:    = followed by a colon

\1   = the first remembered pattern (the first field)
::   = followed by 2 literal colons

s/:[^:]*:/::/

Nope. You can have comments in a sed script, but not within a regular expression. What you are asking is possible with perl if you use the /x regular expression modifier.

Regards,
Alister

---------- Post updated at 10:54 PM ---------- Previous update was at 10:49 PM ----------

s/:[^:]*/:/ would work just as well, unless it's necessary to prevent the last field in a line without a trailing colon from matching, or even s/[^:]*//2 .

REgards,
Alister

That is what I would think too, but this does not work like that everywhere.. This works wiith GNU sed and sed on AIX7 and with regular sed on Solaris, but not with /usr/xpg4/bin/sed on Solaris nor with sed on HPUX and OSX and some other UNIX flavor.

In those cases where it does not work, the desired effect was obtained when s/[^:]*//3 was used instead (and for the 3rd field s/[^:]*//5 and so on).

How can this be? What I think this may have to do with how the respective regex engines interpret a zero match after a previous match. The first match of

echo aaa:bbb:ccc:ddd:eee:fff | sed 's/[^:]*//'

renders

:bbb:ccc:ddd:eee:fff