SED: Can't Repeat Search Character in SED Output

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# 8  
Old 09-16-2009

Definitely I can tell you, what is the logic mistake in your code.

Your code is,
sed '
t loop
s/\n//g' a

In your code,

s/\(\n>[^>]*\)_\([^>]*<\/\)/\1_Q\2/ this line substitutes _ to _Q from right side ( not from left side).

1. In the first iteration, it will substitute This_is_a_test to This_is_a_Qtest
2. In 2nd iteration, it will substitute This_is_a_Qtest to This_is_a_QQtest
3. In the third iteration, it will substitute This_is_a_QQtest to This_is_a_QQQtest
and it will go keep on like this, it never ends.....

This is the reason that your solution works when you replace only with Q, not with _Q.

You can see how the sed substitues,by executing the following code....
sed '
s/\(\n>[^>]*\)_\([^>]*<\/\)/\1_Q\2/p ## prints the pattern space.
t loop
s/\n//g' input

Hope this clarifies your question...
# 9  
Old 09-17-2009
Additional Question

Thanks for the explanations and sample script showing that I was creating an infinite loop!

I'm not sure if I need to start a new thread, but I noticed that the code suggested is unable to accomplish the task if the second tag is on a new line. For example:
sed '
t loop
s/\n//g' a

Do I need to have separate sed lines: one to find cases that fit the normal model where the start and end tags are on the same line and a second sed command that looks for those instances in which the line contains a start tag but ends with a newline character?

---------- Post updated 09-17-09 at 11:33 AM ---------- Previous update was 09-16-09 at 05:14 PM ----------

This may not be very elegant, but it seems to work:

#Script goal: replace all underscores between tags with "_Q"
#The first section handles lines containing both a start and end tag.
sed '
/\n[^<_]*_/b loop
# The second section handles lines only containing a start tag.
/\n[^<_]*_/b loop
s/\n//g' test1.htm

Thanks again for helping me get through this!
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