Use Powershell to comment out a 'codeblock' in a text file?

This is a bit tricky, as you're trying to find that whole section, and then add comment markers to all lines in it. I'd probably write an ad-hoc parser with switch -regex if your structure allows for it (counting braces may make things more robust, but is also a bit harder to get right for all cases). If the code is regular enough you can perhaps reduce it to the following:

stage('inrichting'){
    steps{
        ... some amount of lines that don't contain braces
    }
}

and we can then check for occurrence of the two fixed lines at the start and eventually two lines with closing braces:

 foreach ($file in $list) {
    # lines of the file
    $lines = Get-Content $file
    # line numbers to comment out
    $linesToComment = @()
    # line number of the current block to comment
    $currentStart = -1
    # the number of closing braces on single lines we've encountered for the current block
    $closingBraces = 0

    for ($l = 0; $l -le $lines.Count; $l++) {
        switch -regex ($lines[$l]) {
            '^\s*stage\('inrichting'\)\{' {
                # found the first line we're looking for
                $currentStart = $l
            }
            '^\s*steps\{' {
                # found the second line, it may not belong to the same block, so reset if needed
                if ($l -ne $currentStart + 1) { $currentStart = -1 }
            }
            '^\s*}' {
                # only count braces if we're at the correct point
                if ($currentStart -ne -1) { $closingBraces++ }
                if ($closingBraces -eq 2) {
                # we've reached the end, add the range to the lines to comment out
                $linesToComment += $currentStart..$l
                $currentStart = -1
                $closingBraces = 0
                }
            }
        }
    }

    $commentedLines = 0..($lines.Count-1) | % {
      if ($linesToComment -contains $_) {
        '//' + $lines[$_]
      } else {
        $lines[$_]
      }
    } | Set-Content $file

 }

Untested, but the general idea might work.

Update: fixed and tested