Find array elements which values are not part of another array PowerShell

Question

I have two arrays

• $adg - (A list of AD groups)

• $dbs - (A list of database names)

Lets say I use this command $adg -match $dbs.i

The output will be all the AD groups which have the string $dbs in its name.

However, I am aiming to find the DBs in which are not part of the AD groups array.

Eg:

$adg = @("Dev22_Owner","Test49_Owner","Rocket_Owner")
$dbs = @("Dev22", "Confidential", "InternDB", "Rocket", "Test49")

What approach should I take to get the output:

Confidential

InternDB

I tried $dbs | Where $adg -notmatch $dbs.i but there is no output.

Answer 1

I would first remove the unecessary user part from the ad group:

$groups = $adg | ForEach-Object {
    $_ -replace '_.*?$'
}

Then you can use the Where-Object cmdlet with the -notin operator to filter them:

$dbs | Where-Object { $_ -notin $groups }

Output:

Confidential
InternDB

Answer 2

To offer a more concise PSv3+ alternative to Martin Brandl's helpful answer:

PS> (Compare-Object ($adg -replace '_Owner$') $dbs).InputObject
Confidential
InternDB

• ($adg -replace '_Owner$') returns a new array with copies of the input string that have _Owner suffix stripped from them.

• Compare-Object compares the stripped array with the other array and returns objects that represent the differences between the two arrays.

• Accessing the .InputObject on the difference objects returns the values that differ.

  • Note: If $dbs contained items that aren't in the stripped $adg array, the above would return a union of all differing items, irrespective of which side they're unique to; to distinguish the sides / limit to a side, you'd have to use the .SideIndicator property (value => indicates values exclusive to the right side, <= those exclusive to the left side).

---

As for what you tried:

$adg -match $dbs.i doesn't work as intended and is effectively the same as $adg -match '', which invariably returns all $adg items.

The reason is that array $dbs has no .i property, so the expression evaluates to $null, which on the RHS of match is coerced to a string and therefore is converted to the empty string.

Generally, the RHS of -match does not support arrays - only a single regular expression.

If you do supply an array, it is implicitly converted to a string by joining the elements to form a space-separated list; e.g. array 1, 2 is coerced to '1 2' and '1 2' -match (1, 2) therefore evaluates to $True.