Sed

anandg111 · 08-08-2013, 05:26 AM

can somebody please explain the repeated pattern (&) in sed with propper example.

druuna · 08-08-2013, 05:47 AM

Quote:

Originally Posted by anandg111

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

colucix · 08-08-2013, 05:51 AM

& 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!