With sed you can select all lines in a file between two patterns(including the two lines containing PAT1 and PAT2) and replace them like so:
sed '/PAT1/,/PAT2/c \REPLACEMENT' file
REPLACEMENT is text, which has each embedded newline preceded by a backslash.
Notice that if PAT1 or PAT2 contain foreword slashes / then either each one of them must be escaped with a backslash \/ or the delimiter must be changed to something other than / and in which case you need to escape the first of each delimiter sequence when the c \ command or any other sed command is used e.g. you can change the delimiter to _ like so:
sed '\_PAT1_,\_PAT2_c \REPLACEMENT' file
So you can do:
sed '/<validation>/,/<\/validation>/c \<other-tag>\n<hello/>\n<more>false</more>\n</other-tag>' file
to edit this:
<xml-content>
<validation>
<timeout>2880</timeout>
<subject>example</subject>
<required>true</required>
</validation>
</xml-content>
to this:
<xml-content>
<other-tag>
<hello/>
<more>false</more>
</other-tag>
</xml-content>
Notice that the seds option -i is needed if you want to edit the original file in place ... Test without it first then use it with a backup suffix e.g. -i.back to save a backup copy of the original file with .back extension i.e. file.back like so:
sed -i.back '/PAT1/,/PAT2/c \REPLACEMENT' file
Notice as well that there are tools made especially to edit XML files that you can use to update,delete or insert XML elements ... See How do I replace multiple fields in multiple XML files? as an example ... Those tools however might require a rather more complex command script than what is required forsed in some use cases like yours ... I understand why you asked for a sed solution and would myself prefer sed as well over specialized XML tools for such task ... Plus sed would be more portable of course.
