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Old 08-08-2013, 05:26 AM   #1
anandg111
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Sed - the repeated pattern (&)


can somebody please explain the repeated pattern (&) in sed with propper example.
 
Old 08-08-2013, 05:47 AM   #2
druuna
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Quote:
Originally Posted by anandg111 View Post
can somebody please explain the repeated pattern (&) in sed with propper example.
The & has nothing to do with repeated patterns.
Quote:
& Replaced by the string matched by the regular expression.
If you use something like this:
Code:
sed 's/foo/&bar/' infile
The & is replaced with the regular expression (foo in this case) in the replace part.
Code:
$ cat infile
foo bar

$ sed 's/foo/&bar/' infile 
foobar bar
The above can be rewritten as:
Code:
sed 's/foo/foobar/' infile
Here's another example using a regular expression:
Code:
$ cat infile
foo bar

$ sed 's/.*/& foobar/' infile 
foo bar foobar
The above can also be written as (there are probably more ways):
Code:
sed 's/foo bar/foo bar foobar/' infile

or

sed 's/$/ foobar/' infile
 
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Old 08-08-2013, 05:51 AM   #3
colucix
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& is substituted by the matched string. It's useful when you want to preserve the string matched by a regular expression, but you don't know what exactly is a priori. Suppose you want to embed a number in a string within parentheses. If you know what number is it you should do simply:
Code:
$ echo "My birth year is 1968." | sed 's/1968/(1968)/'
My birth year is (1968).
However if you don't know what the number is, you cannot put it in the replacement string. In this case the & pattern is your friend:
Code:
$ echo "My birth year is 1974." | sed 's/[0-9]\+/(&)/'
My birth year is (1974).
$ echo "My birth year is 1935." | sed 's/[0-9]\+/(&)/'
My birth year is (1935).
Basically it is the whole string matched by the regular expression, that you need to repeat in the replacement string without knowing what is it.

Edit: too late!
 
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