ConvertTo-Json and ConvertFrom-Jason with special characters

I decided to not use Unscape, instead replace the unicode \uxxxx characters with their string values and now it works properly:

$fileContent = 
@"
{
    "something":  "http://domain/?x=1&y=2",
    "pattern":  "^(?!(\\`|\\~|\\!|\\@|\\#|\\$|\\||\\\\|\\'|\\\")).*"
}
"@

$fileContent | ConvertFrom-Json | ConvertTo-Json | %{
    [Regex]::Replace($_, 
        "\\u(?<Value>[a-zA-Z0-9]{4})", {
            param($m) ([char]([int]::Parse($m.Groups['Value'].Value,
                [System.Globalization.NumberStyles]::HexNumber))).ToString() } )}

Which generates the expected output:

{
    "something":  "http://domain/?x=1&y=\\2",
    "pattern":  "^(?!(\\|\\~|\\!|\\@|\\#|\\$|\\||\\\\|\\'|\\\")).*"
}

If you don't want to rely on Regex (from @Reza Aghaei's answer), you could import the Newtonsoft JSON library. The benefit is the default StringEscapeHandling property which escapes control characters only. Another benefit is avoiding the potentially dangerous string replacements you would be doing with Regex.

This StringEscapeHandling is also the default handling of PowerShell Core (version 6 and up) because they started to use Newtonsoft internally since then. So another alternative would be to use ConvertFrom-Json and ConvertTo-Json from PowerShell Core.

Your code would look something like this if you import the Newtonsoft JSON library:

[Reflection.Assembly]::LoadFile("Newtonsoft.Json.dll")

$json = Get-Content -Raw -Path file.json -Encoding UTF8 # read file
$unescaped = [Newtonsoft.Json.Linq.JObject]::Parse($json) # similar to ConvertFrom-Json

$escapedElementValue = [Newtonsoft.Json.JsonConvert]::ToString($unescaped.apiName.Value) # similar to ConvertTo-Json
$escapedCompleteJson = [Newtonsoft.Json.JsonConvert]::SerializeObject($unescaped) # similar to ConvertTo-Json

Write-Output "Variable passed = $escapedElementValue"
Write-Output "Same JSON as Input = $escapedCompleteJson"