The name kind of covers it. For example https://unittest.some.company.com should have an application pool of unittest.some.company.com. A second example would be https://unittest.some.company.com/subsitea/ should have an application pool of unittest.some.company.com_subsitea.

WebAdmin.ConvertTo-UrlBasedAppPoolName.ps1

<#
.SYNOPSIS
	Enforces the formatting standards for application pool names.

	This should be used to figure out the application pool name before creating an
	new one. The name is also used to create unique ARR rule names.

.PARAMETER Url
	The url to parse and convert to our standardized app pool name.

.PARAMETER Environment
	If an environment is also passed, the url will be run through Get-WebEnvironmentUri
	before being parsed/converted.

.LINK
	Get-WebEnvironmentUri

.EXAMPLE
	$url = "http://unittest.{env}.place.something.com"
	$env = "dev"

	$appPoolName = ConvertTo-UrlBasedAppPoolName -Url $url -Environment $env
#>
Function ConvertTo-UrlBasedAppPoolName {
[CmdletBinding()]
Param (
	[Parameter(Mandatory = $true)]
	[string] $Url
)

	$parseUrl = $Url
	$m = "(https?://)?(.*)"
	if($parseUrl -match $m) {
		$hostPath = $Matches[2]
	}
	
	$hostPath = $hostPath.Replace("/","_")
	# this prevents http://aaa.sa.ucsb.edu/ from becoming aaa.sa.ucsb.edu_
	$pathLen = $hostPath.Length;
	if($pathLen -gt 0) {
		if($hostPath[$pathLen - 1] -eq "_") {
			$hostPath = $hostPath.Substring(0, $pathLen - 1);
		}
	}

	return $hostPath.ToLower()
}