ConvertFrom-Hex.ps1

<#
.SYNOPSIS
Convert a string of hexadecimal digits into a byte array.

.INPUTS
System.String of hex digits.

.OUTPUTS
System.Byte[] of bytes parsed from hex digits.

.FUNCTIONALITY
Data encoding

.LINK
https://docs.microsoft.com/dotnet/api/system.numerics.biginteger.parse#System_Numerics_BigInteger_Parse_System_String_System_Globalization_NumberStyles_

.LINK
https://docs.microsoft.com/dotnet/api/system.array.reverse#System_Array_Reverse_System_Array_

.EXAMPLE
ConvertFrom-Hex.ps1 'EF BB BF'

239
187
191

.EXAMPLE
[text.encoding]::UTF8.GetString((ConvertFrom-Hex.ps1 0x25504446))

%PDF

.EXAMPLE
'{0:X2} {1:X2} {2:X2}' -f (ConvertFrom-Hex.ps1 c0ffee)

C0 FF EE
#>

#Requires -Version 3
[CmdletBinding()][OutputType([byte[]])] Param(
# A string of hex digits.
[Parameter(Position=0,Mandatory=$true,ValueFromPipeline=$true)][string] $InputObject
)
Process
{
	$hex = $InputObject -replace '\A0x|[^0-9A-Fa-f]+',''
	[byte[]] $value = [bigint]::Parse("00$hex",'HexNumber').ToByteArray()
	[array]::Reverse($value)
	while($value.Length -gt ($hex.Length / 2)) {$null,$value = $value}
	return $value
}