The most succinct and elegant hardly-hard-coded [standard]RomanNumeral converter you've probably ever seen

romannumerals.ps1

Filter FromRoman {
	$output = 0
	
	if ($_ -notmatch '^(M{1,3}|)(CM|CD|D?C{0,3}|)(XC|XL|L?X{0,3}|)(IX|IV|V?I{0,3}|)$') {
		throw 'Incorrect format'
	}
	
	$current = 1000
	$subtractor = 'M'
	$whole = $False
	$roman = $_
	'C','D','X','L','I','V',' ' `
	| %{
		if ($whole = !$whole) {
			$current /= 10
			$subtractor = $_ + $subtractor[0]
			$_ = $subtractor[1]
		}
		else {
			$subtractor = $subtractor[0] + $_
		}
		
		if ($roman -match $subtractor) {
			$output += $current * (4,9)[$whole]
			$roman = $roman -replace $subtractor,''
		}
		if ($roman -match ($_ + '{1,3}')) {
			$output += $current * (5,10)[$whole] * $Matches[0].Length
		}
	}
	
	$output
}

Filter ToRoman {
	$output = ''
	
	if ($_ -ge 4000) {
		throw 'Number too high'
	}
	
	$current = 1000
	$subtractor = 'M'
	$whole = $False
	$decimal = $_
	'C','D','X','L','I','V',' ' `
	| %{
		$divisor = $current
		if ($whole = !$whole) {
			$current /= 10
			$subtractor = $_ + $subtractor[0]
			$_ = $subtractor[1]
		}
		else {
			$divisor *= 5 
			$subtractor = $subtractor[0] + $_
		}
		
		$multiple = [Math]::floor($decimal / $divisor)
		if ($multiple) {
			$output += [string]$_ * $multiple
			$decimal %= $divisor
		}
		if ($decimal -ge ($divisor -= $current)) {
			$output += $subtractor
			$decimal -= $divisor
		}
	}
	
	$output
}

>'XIX','IV','','MMCDLXXIX','MMMI' | FromRoman
19
4
0
2479
3001

>19,4,0,2479,3001 | ToRoman
XIX
IV

MMCDLXXIX
MMMI