Powershell to list free domain computers - powershell

Question

I'm using PowerShell to get a lit of all our domain computers, and then look for ones that are skipped, so we can easily get free computer names within this name range.
For example:
Get-ADComputer -Filter {Name -like "PC016*"} | Sort-Object | select Name
This gets me all our PCs starting with "PC016", and it works. From here I want to list all the skipped names.
For example, if I have this output:
PC016225
PC016226
PC016228
PC016229
I want Powershell to list the skipped item (PC016227).
How can I do this?

Answer 1

Remove the prefix from the names so you're left with the numerical suffix (eg. 225), then sort the resulting values to easily locate the lowest and highest values, and then simply output any number in between that isn't in the original list:
# Define computer name prefix
$prefix = "PC016"
# Create regex pattern for the prefix + a pattern to match only computer names ending with numbers
$prefixPattern = "^$([regex]::Escape($prefix))"
$namePattern = "${prefixPattern}\d+$"
# Query the directory for computer objects with the given naming prefix,
# filter the result so we only store computers with a numerical suffix
$computerNames = Get-ADComputer -Filter "Name -like '${prefix}*'" |Where-Object Name -match $namePattern |ForEach-Object Name
# Extract the numerical suffix and convert to a numeric type
$values = #($computerNames -replace $prefixPattern) -as [int[]]
# Sort the values
$values = $values |Sort-Object
# Now generate the range of values from the smallest to the biggest,
# filter out any values that are already found in the existing list
$skipped = $values[0]..$values[-1] |Where-Object { $_ -notin $values }
# ... and finally attach the prefix again and output the new possible names
$skipped |ForEach-Object { -join $prefix,$_ }