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# 1  
Old 09-14-2009
SED: Can't Repeat Search Character in SED Output

I'm not sure if the problem I'm seeing is an artifact of sed or simply a beginner's mistake. Here's the problem: I want to add a zero-width space following each underscore between XML tags. For example, if I had the following xml:

<MY_BIG_TAG>This_is_a_test</MY_BIG_TAG>

It should look like this after I run my sed script:

<MY_BIG_TAG>This_​is_​a_​test</MY_BIG_TAG>

To accomplish this, I found an example from the web and modified it for my purposes. Unfortunately, the example does not allow me to search for the underscore and then re-use it in the output. The script works fine if I just want to search on the underscore character and replace it with a different character (in this case, the zero-width space); however, as soon as I try to search for the underscore and replace it with that same underscore followed by a zero-width space, sed stalls and never completes.

Here's the script:

#/usr/bin/sh

sed 's/>[^>]*<\//\n&/g #This isolates strings in which I'm interested by inserting newline characters.
:loop
s/\(\n>[^>]*\)_\([^>]*<\/\)/\1_\​\2/ #This is supposed to replace "_" with "_​" but it does not; instead, it stalls sed.
t loop
s/\n//g' 1.xml #This removes the newline characters

This doesn't look like a problem with the underscore character itself since I have the same problem no matter what character I search on: I'm unable to find a character and replace it with the same character followed my something new.

This seems like such a basic sed feature that I'm inclined to think I'm doing something wrong.

Any ideas?

Thanks,

Rob
# 2  
Old 09-14-2009
I don't see any difference between the given input and desired output.

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# 3  
Old 09-14-2009
Code correction

I made a mistake when I posted my code example. I neglected to show that I wanted to replace all underscores with that same underscore followed by a zero-width space. Here is the corrected example (I added "​" in the sed loop:

<code>

#/usr/bin/sh

sed 's/>[^>]*<\//\n&/g #This isolates strings in which I'm interested by inserting newline characters.
:loop
s/\(\n>[^>]*\)_\([^>]*<\/\)/\1_​​\​\2/ #This is supposed to replace "_" with "_​​​" but it does not; instead, it stalls sed.
t loop
s/\n//g' 1.xml #This removes the newline characters

</code>

---------- Post updated at 03:09 PM ---------- Previous update was at 02:44 PM ----------

The forum wordprocessing tool has been removing my example of the zero-width space. When I try to type an ampersand followed by the pound sign and "8203;" it is rendered as a blank space by the forum editor.

Perhaps the moderator could help with the the required escape characters in this editor?

In the meantime, I'll provide an example using different characters that aren't problematic for the text editor. Here's an example where I want to replace all underscores with that same underscore followed by the letter "q":

Code:
#/usr/bin/sh
 
sed 's/>[^>]*<\//\n&/g #This isolates strings in which I'm interested by inserting newline characters.
:loop
s/\(\n>[^>]*\)_\([^>]*<\/\)/\1_Q​​\​​\2/ #This is supposed to replace "_" with "_​​​​Q" but it does not; instead, it stalls sed.
t loop
s/\n//g' 1.xml #This removes the newline characters

Here is the code for the source file (1.xml)

Code:
<MY_BIG_TAG>This_is_a_test</MY_BIG_TAG>

My desired result with this revised example using "Q" is the following:

Code:
<MY_BIG_TAG>This_Qis_Qa_Qtest</MY_BIG_TAG>

The code above works fine as long as I don't repeat the underscore in the output; in other words, if I replace the underscore with only the letter "Q" sed is able to complete.

Is there a way I can have sed repeat the underscore followed by the letter "Q"?

Thanks,

Rob
# 4  
Old 09-15-2009
To replace "_" with "_Q" with sed:

Code:
sed 's/_/_Q/g' file

# 5  
Old 09-15-2009
Hope this works for you... Slight modification from your solution:
Code:
Input:
<MY_BIG_TAG>This_is_a_test</MY_BIG_TAG>

Code:
sed '
s/\(<[^>]*>\)\([^>]*\)\(<[^>]*>\)/\1\n\2\3/g 
:loop
s/\n\([^<_]*\)_/\1_Q\n/g 
/\n[^<_]*_/b loop
s/\n//g' a

Output:
<MY_BIG_TAG>This_Qis_Qa_Qtest</MY_BIG_TAG>

Explanation :

1. s/\(<[^>]*>\)\([^>]*\)\(<[^>]*>\)/\1\n\2\3/g
This replaces like <MY_BIG_TAG>\nThis_is_a_test<MY_BIG_TAG>
2. starts loop
3. After \n till < arrives, substitute all underscore to _Q
4. Again checks if the same pattern appears, if it is, go through the loop again.
5. Atlast replace \n with the empty ( which we replaced in line 1).
# 6  
Old 09-15-2009
The code you suggested worked very well! It replaces only the underscores betwen the tags, which is want I wanted.

Code:
sed '
s/\(<[^>]*>\)\([^>]*\)\(<[^>]*>\)/\1\n\2\3/g 
:loop
s/\n\([^<_]*\)_/\1_Q\n/g 
/\n[^<_]*_/b loop
s/\n//g' a

What I'd like to know is what are the key differences in your script that enables sed to reuse the underscore, whereas in mine, sed completely hangs if I try to use the "\1_Q" (but works if I just use "\1Q").

Any ideas?

Last edited by rhetoric101; 09-15-2009 at 05:12 PM.. Reason: Forgot closing parenthesis
# 7  
Old 09-16-2009
Quote:
Originally Posted by rhetoric101
sed completely hangs if I try to use the "\1_Q" (but works if I just use "\1Q").
You basically search for "_" and replace it with "_Q". In the next round of the loop you find what you have just replaced and replace again thereby establishing an infinite loop.

You would have to search for "_<not followed by a Q>" (in regex "_[^Q]*") to avoid that loop.

I hope this helps.

bakunin
 

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