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

Question

I do have my text as follows in my config file

[ {root, [ {child, 123} ]} ]. or []

when the text is [] I am doing as follows

 

 $fileName = Get-ChildItem -Path "$filePath" -Recurse -Filter "fileName.txt"
 $resultMatch = Select-String -Path $fileName.FullName -Pattern "root"

if([string]::IsNullOrWhiteSpace($resultMatch))
{
    $content=''
    $content = "[
    {root,
   [
    {child, 123} // this value may change
   ]}
 ]."

    Clear-Content $newfileName
    Add-Content  $newfileName $content
}

 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

[
 {root,
      {child, 0}, if exists replace the entire line else add
      {child, 1}
  ]}
].