Help with sed code line


 
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# 1  
Help with sed code line

Can anyone tell me what the following line of code does?

Code:
sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g' file1 > file2


Last edited by Scott; 05-04-2013 at 04:41 PM.. Reason: Please use code tags
# 2  
Quote:
Originally Posted by dee campbell
Can anyone tell me what the following line of code does?

Code:
sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g' file1 > file2

This part :

Code:
sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g' file1

uses the "sed" utility to search for a pattern in file "file1" and replace it by something, if it is found. If the pattern is not found, the line will be returned as it is.

This part:

Code:
> file2

redirects the output of the sed command above to another file called "file2". If it exists, the file "file2" will be overwritten, otherwise it will be created.

Now for the search/replace part:

Code:
s                               : search for a pattern
/
2                               : that starts with the character "2"
\([0-2][0-9]\)              : followed by a two-digit number between "00" and "29" both inclusive. Store this two-digit number in a temporary variable called "\1"
\([0-5][0-9]\)              : followed by a two-digit number between "00" and "59" both inclusive. Store this two-digit number in a temporary variable called "\2"
/                                : if such a pattern is found, replace it with
#\1                            : the character "#" followed by the value of the temporary variable "\1" derived above
#\2#                          : followed by the character "#" followed by the value of the temporary variable "\2" derived above
/g                              : and perform this search/replace process for each such pattern in each line of the file

Here's an example of the working of the sed command. I've omitted the redirection part at the end:

Code:
$
$ cat f20
blah 20000 blah 20059 blah
blah 23456 blah 22959 blah
blah 29988 blah 39988 blah
$
$ sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g' f20
blah #00#00# blah #00#59# blah
blah 23456 blah #29#59# blah
blah 29988 blah 39988 blah
$
$

(a) Line 1
Both the numbers in the first line matched the pattern, hence they were replaced.

(b) Line 2
The first number in the second line did not match the pattern, because "2" was not followed by a number between "00" and "29" (both inclusive). It was followed by "34", so the first number was left as it is. The second number matched the pattern and was suitably replaced.

(c) Line 3
None of the two numbers matched the pattern, so the line was returned as it was.
# 3  
If you look at the examples closely, along with the explanation durden_tyler provided, you'll see what the code does.
Code:
$ echo 23344 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
23344
$ echo 22344 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
#23#44#
$ echo 72344 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
72344

Code:
$ echo 22364 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
22364
$ echo 22354 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
#23#54#
$ echo 12354 | sed 's/2\([0-2][0-9]\)\([0-5][0-9]\)/#\1#\2#/g'
12354

 

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