Regex or best way to simplify the code to extract and append

I do have my text as follows in my config file

when the text is [] I am doing as follows

If root was found i need to check for child child if not exists I need to append else add.

I am doing as follows $resultMatch = Select-String -Path $fileName.FullName -Pattern "{child,"

I am thinking of a better way to do it, so any one having any idea.

The entire file will have as follows