Replace a text part in textfile

Goal

Replace the text part between ### Test: and ###.

Regex

(?<=<YourStartString>)(.|\n)*(?=<YourEndString>)

Example for a text part:

### Test
This Text will
changed with the following
PowerShell Script
###

Regex for the example text part below:

(?<=\#\#\#\.Test)(.|\n)*(?=\#\#\#)

PowerShell Script

File to change - Before the PowerShell Script

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, 
sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, 
sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, 
no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, 
consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, 
sed diam voluptua. 
At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, 
no sea takimata sanctus est Lorem ipsum dolor sit amet.

### Test
This Text will
changed with the following
PowerShell Script
###

PowerShell Script to change text part in a file

$FileToChangePath = "C:\Temp\FileToChange.txt"
$FileToChange = Get-Content $FileToChangePath -Raw
 
$TemplateTextPartPath = Get-Content "C:\Temp\TemplateTextPart.txt" -Raw
 
$RegexSearch = "### Test" #Search the head of text part
$RegexReplace = "(?<=\#\#\#\.Test)(.|\n)*(?=\#\#\#)"
 
If ($FileToChange -match $RegexSearch) {
    Write-Host "The text part in the file has been changed." -ForegroundColor Green
    $ChangeTextPart = $FileToChange -replace $RegexReplace, ("`r`n" + $TemplateTextPartPath + "`r`n")
    Set-Content -Path $FileToChangePath -Value $ChangeTextPart
} Else {
    Write-Host "The text part in the file has been added." -ForegroundColor Yellow
    Add-Content -Path $FileToChangePath -Value ("`r`n" + "`r`n" + "`r`n" + "`r`n" + "### Test" + "`r`n" + $TemplateTextPartPath + "`r`n" + "###" ) -PassThru
}

Text part in the file has been changed - After the PowerShell Script

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, 
sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, 
sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, 
no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, 
consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, 
sed diam voluptua. 
At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, 
no sea takimata sanctus est Lorem ipsum dolor sit amet.

### Test
This Text is
the changed text
with the PowerShell Script
###

Regex101: https://regex101.com/