twantrd

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Hi,

I have a log file and I want to remove a string for parsing purposes. My log file looks like this

I want to remove the username, U=root. However, the username can be anything else as well such as Sandy, Mike, Joe, etc. Therefore, I want to remove U=* but NOT the rest. The result should be

My sed syntax is
So, I'm trying to match everything after U= and stop at a blank line but it's not working right. Can someone shed some light? Thanks!

-twantrd

druuna

Hi,

This should work:

sed 's/ U=.* P/ P/' <infile>

It assumes that there's always a P= entry after U=<user>.

Hope this helps.

spirit receiver

... and if you want to stop at space characters instead, you can use

makyo

Hi, twantrd.
You received 2 good specific answers for the problem.

One of the principles I try to get across to students in my classes is that regular expressions are by design greedy. They will match the longest possible string. So if you need to match fewer characters, you need to supply a constraint of some kind. As you saw, that took the form of additional text that followed the .* part of the pattern.

That being said, however, it is easy to forget details like that when we're busy with a million other things ... cheers, makyo (26)

twantrd

Thanks everyone!

Druuna, I understand your syntax. Makes sense. Thanks!

Spirit receiver, can you explain how yours works? I understand that [[:space:]] means an actual space but what does [^[:space:]]* mean? I thought [^ ]* will match everything EXCEPT a space. So, wouldn't the syntax be like this:

sed 's/ U=*[^[:space:]]//' - Match everything after U= but do not match space.

Obviously, this doesn't work as I've just tried it. Thanks.

-twantrd

druuna

Hi,

Strange it doesn't work (it does at my end).

About the [^[:space:]]* construct: This will allow anything except (the ^) whitespace.

And yes, you could also write: [^ ]*. But if there's a tab between root and P= instead of a 'real' space, [^ ]* will be greedy and the P= part will also be targeted (wherever the first 'real' space is found).

Hope this clears things up a bit.

soggycornflake

I think you're confusing the shell meta-character * with the regex repetition operator *.

spirit receiver's pattern '...[^[:space:]]*...' says

[ match a character class (matches 1 character)
^ not (i.e. a character that doesn't match one of the following)
[:space:] match a space character
] closing outer bracket
* repeated zero or more times.

i.e. match any sequence of characters (including nothing) that do not contain a space (or newline/cr/etc).

Whereas your string '...U=*[^[:space:]]...' matches U followed by any number of = characters, followed by any character except a space.

Bear in mind that in regular expressions, * doesn't match anything on its own (nor does ?, + etc), these are repetition operators which apply to the preceding pattern.

twantrd

Ahh ok that makes sense. Thanks for the clarification!

-twantrd