Extract a substring with a regular expression in PowerShell

One way is:

$a = "[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\DNS Server\anyLongString]"
$a -match '([^\\]*)]$'
$matches[1]
anyLongString

Another way to do it is using fewer lines of code:

$a = "[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\DNS Server\anyLongString]"
$a -Replace '^.*\\([^\\]+)]$', '$1'