Parses a file full of user name and user IDs and outputs only the domain names found.

ParseDomains.ps1

# Parses display name w/ user IDs (e.g., "John Doe [mydomain\jdoe]") to get the domain name
# Handles multiple per line if separated by semicolons
# Input is the filename that contains the rows of display names with user IDs
# Outputs only the domain name (does not output duplicates for the same row)
$currentPath=Split-Path ((Get-Variable MyInvocation -Scope 0).Value).MyCommand.Path
$filename = Join-Path $currentPath $args[0]
foreach ($line in [System.IO.File]::ReadLines($filename)) {
    $users = $line.ToLower().Split(';')
    $domains = new-object System.Collections.ArrayList
    foreach ($user in $users)
    {
        $domainName = $user # Default domain name to entire user string as a fallback
        $index = $user.IndexOf('[')
        if ($index -ge 0)
        {
            # We know where the domain name starts, now find out where it ends
            $userIdPart = $user.Substring($index + 1)
            $index = $userIdPart.IndexOf("\")
            if ($index -ge 0)
            {
                # We know where the domain ends - so get it
                $domainName = $userIdPart.Substring(0, $index)
            }
        }
        if ($domainName.length -gt 0 -and -not $domains.Contains($domainName))
        {
            # Add the domain name to our array, but only if it isn't a duplicate
            $domains.Add($domainName) | out-null
        }
    }

    # Sort the domain names for this row
    $domains.Sort()

    # Join the domain names for this row and have them separated by a semicolon
    $domainsForRow = [System.String]::Join(";", $domains.ToArray([System.Type]::GetType("System.String")))

    # Output the domain names for this row
    Write-Output $domainsForRow
}