What I think I'm needing is an expression to exclude anything outside of the 3rd and 4th instances of a character, backlashes in this case. Does that sound right?

The problem I'm trying to solve is how to get the ShareName from a given UNC path in PowerShell.

Find: "Share01" in example source string:

\\FQDN\Share01\Directories\...\...\...\...

PowerShell's Split-Path is proving to be a bit limitied in this regard, and RegEx is ...hard!

Hm I'm really late to this party, but I will suggest an alternative: Use the [Uri] type:

$unc = '\\FQDN\Share01\Directories\...\...\...\...'
$upath = [Uri]$unc

Testing if the path really is a UNC path:

$upath.IsUnc

Getting the Share

$upath.Segments[1]            # Returns Share01/
$upath.Segments[1].Trim('/')  # Trims the trailing /

Or a one liner with no escaping or trimming:

[System.IO.Path]::GetPathRoot($unc).Split('\')[-1]