PowerShell Dobule Escape string with text "$__VAR__" - powershell

Question

One of my scripts can be stripped down to the following code.
function Replace
{
[CmdletBinding()]
Param
(
[Parameter(Mandatory, Position=0)]
[string]
$LiteralPath,
[Parameter(Mandatory, Position=1)]
[string]
$Expression,
[Parameter(Mandatory, Position=2)]
[string]
$Replacement
)
Get-Content $LiteralPath | ForEach-Object {$_ -replace $Expression, $Replacement } | Set-Content $LiteralPath + ".temp"
}
An example call to the script would be
Replace ".\MyFile.txt" "^#define abc.*" "#define abc 1"
I have run into a situation where the string I need to find and replace contains both dollar signs and underscores. The dollars signs must be escaped to prevent PowerShell from expanding the variable. One string contains a dollar sign followed by an underscore. This is causing an issue because PowerShell is not expanding the variable name but then is expanding the $_ piping variable. How can I prevent PowerShell from expanding both variable name and piping token.
This is an example cal to the function with a string I need to escape.
Replace ".\MyFile.txt" "^\#\`$__LIBRARY_DIR\\prj.gpj" "`$__LIBRARY_DIR\prj.gpj"
In this example the line of text which reads #$__LIBRARY_DIR\prj.gpj is getting changed to #$__LIBRARY_DIR\prj.gpj_LIBRARY_DIR\prj.gpj. I am looking for the text to be changed to $__LIBRARY_DIR\prj.gpj
Notice the $_ is expanded which I do not want it to expand. I have tried adding more escape characters but that only causes them to appear in the file. How can the string be escaped to prevent $_ from expanding?

Answer 1

In powershell, if you don't want variables to expand in your string, use 'single quotes' instead if "double quotes", that saves you the trouble of escaping the $ sign with backticks.
Now in your case you have the additional challenge, that the -replace operator will also want to expand expressions that start with the $ sign in the replacement string, regardless of the types of quotes that you use.
To tell -replace that you really want to see that $ in your replacement string, you need to write $$:
'#$__LIBRARY_DIR\prj.gpj' -replace '^#\$__LIBRARY_DIR\\prj.gpj','$$__LIBRARY_DIR\prj.gpj'
Note: As others have correctly pointed out in the comments, if your task is to strip expressions from a leading #, you can do that in a more simple way:
'#$__LIBRARY_DIR\prj.gpj' -replace '^#'
Or alternatively with the good old "trim":
'#$__LIBRARY_DIR\prj.gpj'.TrimStart('#')