Find characters between Greater Than and Less Than signs

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

Details

Uses a regex expression with find all characters that match the expression.

Example

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

FirstName