Find characters between Parentheses ()

$string = "LastName, FirstName ([samaccountname])"
[Regex]::Matches($string, '(?<=\()(.*?)(?=\))').Value
Details
Uses a regex expression with find all characters that match the expression.

Example
PS C:\> $string = "LastName, FirstName ([samaccountname])"
>> [Regex]::Matches($string, '(?<=\()(.*?)(?=\))').Value

[samaccountname]
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