Is there a way to use regex to insert a line into a specific group in a file?

Quote:

but that just replaces the whole thing with hello which is not what I want. I need it to insert hello in the middle of the Home group.

Because you need to define 'groups' in your RegEx, the groups are what you want to keep from original string.
Try something like:

(<Home />\n)([^.]+</Home>)

Just a few interesting links to help building and debugging RegEx.
Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
RegExr: Learn, Build, & Test RegEx[^]
Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx:
Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]
This site also show the Regex in a nice graph but can't test what match the RegEx:
Regexper[^]