UNIX for Dummies Questions & Answers

First of all let it be said that I hate sed. Every time I think I've got it figured out it proves me wrong. There, now with that out of the way here's my problem that I'm hoping one of you smart people can solve:

I have a file full of lines like this:

Code:

If x=1 then /* this is an if statement */
/* new line added */ echo "hello"

I want to remove the /* and */ everything between them too but leave the rest of the line intact.

Unfortunately this is proving really difficult since, based on the roughly 15 threads I've read about the topic of sed, both / and * seem to be special characters so using them as the search critiera is proving difficult for me to figure out. This is the closest I've gotten:

Code:

sed 's/*.**/*/g' results.tmp

But this only searches for the * and not both. It also wipes out everything to the right of the first * so that's not going to work.

Any help would be appreciated. Thanks.

The following will work with the data sample you provided. However, it will not work with comments that span multiple lines. If you need something more complicated, you might want to look into using a tool that's familiar with the language in question.

Code:

sed 's|/\*[^*]*\*/||g'

Regards,
Alister

Alister, you're amazing.

Can you do me one more favor and explain the syntax to me so I'll know for next time?

Code:

/\*[^*]*\*/

/ - Begin search pattern

\* - Starts with asterisks * ( It is escaped \* because meta-character)

[^*] - Followed by any character other than ^ asterisks * (It not escaped because meta-characters loose there special meaning inside [ ])

* - Zero or more occurrences

\* - Followed by asterisks *

/ - End search pattern

After that characterization, how can I refuse.

You are correct in your original post when you say that / and * are both special to sed. However, sed's substitution command allows us to choose a delimiter other than /. For example, the following two commands are functionally identical, each replaces every occurrence of foo with bar:

Code:

sed 's/foo/bar/g'
sed 's|foo|bar|g'

In the second instance, I chose a pipe symbol, |, instead of the forward slash. The benefit of this choice is that if we need to use literal forward slashes within the s command, we don't have to precede them with a backslash. To change a/c to d/c, first with the default delimiter and then with the pipe symbol instead:

Code:

sed 's/a\/c/d\/c/g'
sed 's|a/c|d/c|g'

If you can imagine the number of forward slashes in something like a url (http://domain/dir/file), using a different delimiter can improve readability of the command.

While I chose a pipe in those examples, you can choose any character you like except a newline or a backslash. Whatever character you choose, you can still use it within the command, but you'll have to precede it with a backslash.

Breaking down the /\*[^*]*\*/ regular expression:
/\*: forward slash followed by a literal asterisk (which needs to be backslashed since it's normally a special character)
[^*]*: The expression in brackets means anything except an asterisk (which does not have to be backslashed because special characters are not special within the bracket expression). The asterisk that follows the closing bracket is indeed special. As a whole, this fragment means zero or more non-asterisks. This is used to ensure that we don't go past the end of the comment, which ends with a */ sequence.
\*/: This matches the */ sequence at the end of the comment. Backslash escape the asterisk so it's taken literally and follow it with the forward slash (which isn't special here and does not require a preceding backslash because we're using the pipe as a delimiter).

Regards,
Alister

---------- Post updated at 08:25 PM ---------- Previous update was at 08:23 PM ----------

Thank you for helping with the explanation, Yoda. If I had known you were there, I would've saved myself some time. However, you misinterpreted the slashes at the beginning and the end. They are literal characters. The pipes delimit the regular expression.

Regards,
Alister

To which poster's example are you referring to?

Korn0474:

Ignore that post. That's probably a spammer in the making (already reported). That post is simply using words from your first post in this thread.

Regards,
Alister