Q:

regex (vim) for print ... to print(...) for python2 to python3

This post is helpful only if you have strings inside of the print command. 
Now I have tons of sourcecode with a statement such as
print milk,butter

which should be formatted to
print(milk,butter)

And capturing the end of the line with \n was not sucessfull. Any hints?

A:

I am not familiar with 2to3, but from all the comments, it looks like the correct tool for the job.
That said, perhaps we can use this question as an excuse for a short lesson in some vim basics.
First, you want a pattern that matches the correct lines.  I think that ^\s*print\> will do:

^ matches start of line (and $ matches end of line).
\s matches whitespace (space or tab)
* means 0 or more of the previous atom (as many as possible, or "greedy").
print is a literal string.
\> matches end-of-word (zero width). You might use a (literal) space or \s\+ instead.

Next, you need to identify the part to be enclosed in parentheses.  Since * is greedy, .* will match to the end of the line; there is no need to anchor it on the right.  Use \(\s*print\) and \(.*\) to capture the pieces, so that you can refer to them as \1 and \2 in the replacement.
Now, put the pieces together.  There are many variants, and I have not tried to "golf" this one:
:%s/^\(\s*print\)\s\+\(.*\)/\1(\2)

Some people prefer the "very magic" version, where only a-z, A-Z, 0-9, and _ are treated as literal characters; then you do not need to escape the parentheses nor the plus:
:%s/^\v(\s*print)\s+(.*)/\1(\2)

A:

You could use 2to3 and only apply the fix for print statement -> print function.
2to3 --fix=print [yourfiles]

This should automatically handle all those strange cases which won't work with e.g. a vim regex.
If you are missing the 2to3 shell script for some reason, run the lib as a module:
python -m lib2to3 --fix=print [yourfiles]

A:

Since you're in vim already:
:!2to3 --fix=print --write %

