How to insert text between two set patterns with sed?

_{For a sed solution, see further down in this answer.}

Assuming that the a nodes are part of a well formed XML document, and that you would want to append .xhtml to the value of their href tags when the existing values starts with /entry/:

xml ed -u '//a[starts-with(@href, "/entry/")]/@href' \
       -x 'concat(../@href,".xhtml")' file.xml >file-new.xml

This uses XMLStarlet (sometimes installed as xmlstarlet instead of just xml) and it will find the relevant a nodes and append .xhtml to their href attributes regardless where in the document they occur.

The result is saved to a new file here, but you may use xml ed --inplace ... to edit the file in place once you have made sure that it works.

Testing:

$ cat file.xml
<?xml version="1.0"?>
<root>
  <a href="/entry/someFile1"/>
  <a href="/entry/someFile2"/>
  <a href="/entry/someFile3"/>
</root>

$ xml ed -u '//a[starts-with(@href, "/entry/")]/@href' -x 'concat(../@href,".xhtml")' file.xml
<?xml version="1.0"?>
<root>
  <a href="/entry/someFile1.xhtml"/>
  <a href="/entry/someFile2.xhtml"/>
  <a href="/entry/someFile3.xhtml"/>
</root>

Using sed (which you would not use on a well formed XML file usually):

sed 's|<a href="/entry/[^"]*|&.xhtml|g' file.xml

This matches the string <a href="/entry/ followed by any number of characters that are not " (this would be the filename). This whole matching part is then replace with itself and the string .xhtml.

With sed -i, this would make the modification in place.

Testing (on the same file as above):

$ sed 's|<a href="/entry/[^"]*|&.xthml|g' file.xml
<?xml version="1.0"?>
<root>
  <a href="/entry/someFile1.xhtml"/>
  <a href="/entry/someFile2.xhtml"/>
  <a href="/entry/someFile3.xhtml"/>
</root>

sed can be very complicate, but for your needs its easy to use try:

sed -i 's/<a href=".*">/<a href="/some/link/">/g' yourfile.html

The syntax here is easy:

sed -i 's/stringt before replacing/string after replacing/g'

the .* is the wildcard matches anything use it at the needed place

Maybe you should copy the file before using sed. the -i changes your file not creates a new one:

-i [SUFFIX], --in-place[=SUFFIX] edit files in place (makes backup if SUFFIX supplied)

the g at the end replaces all matches in the file

if you want to change only the first match in your file use:

sed -i '0,/<a href=".*">/{s/<a href=".*">/<a href="/some/link/">/}' yourfile.html

same syntax:

sed -i '0,/string before/{s/string brefore/string after/}'