How do edit the last occurrence of a particular string in powershell - powershell

Question

My text file contains G-Code with the code "G94" appearing 5 times at different line numbers.
G94
G94
G94
G94
G94
I need to change the last occurrence of "G94" to
G94
/M16
but I keep getting no edit at all.
I'm trying this:
$text = get-content C:\cncm\g94.txt
$i = 1
$replace = 5 #Replace the 5th match
ForEach ( $match in ($text | select-String "G94" -AllMatches).matches)
{
$index = $match.Index
if ( $i -eq $replace )
{
$text.Remove($index,"G94".length).Insert($index,"G94 n /M16")
}
$i++
}
What am I missing?

Answer 1

$text is an array of strings, how are you calling Remove() without getting an exception? First because Remove() only takes one parameter, second because you can't remove from a fixed length array.
I'm thinking:
$text = get-content C:\cncm\g94.txt
$fifthMatch = ($text | select-string "G94" -AllMatches)[4]
$line = $text[$fifthMatch.LineNumber]
$line = $line.Remove($fifthMatch.index,"G94".length).Insert($fifthMatch.index,"G94 `n /M16")
$text[$fifthMatch.LineNumber] = $line
$text | out-file c:\cncm\g942.txt

Answer 2

Use regexp with negative lookahead on a string that contains the entire file.
Replacing last occurrence in the entire file - (?s) DOTALL mode allows .* to span across new line characters:
$text = [IO.File]::ReadAllText('C:\cncm\g94.txt')
$text = $text -replace '(?s)G94(?!.*?G94)', "G94`n/M16"
Replacing last occurrence in every line - (?m) MULTILINE mode:
$text = [IO.File]::ReadAllText('C:\cncm\g94.txt')
$text = $text -replace '(?m)G94(?!.*?G94)', "G94`n/M16"