Find characters between Brackets [] 
$string = "LastName, FirstName ([samaccountname])"
[Regex]::Matches($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
  |  
Find characters between double quotes “ 
$string = '"LastName, FirstName" ([samaccountname])'
[Regex]::Matches($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

LastName, FirstName
  |  
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
  |  
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]
  |  
Find characters between single quotes ‘ 
$string = "'LastName',  ([samaccountname])"
[Regex]::Matches($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

LastName
  |  
Find none ASCII characters 
$string = "In mathematics, summation (capital Greek sigma symbol: ∑) is the addition of a sequence of numbers."
$string.ToCharArray() | ForEach-Object{
    $char = $_
    try{$ascii = [byte][char]$char}
    catch{"$char - None ascii character" }
}
Details
Breaks the string into a char array and attempts is convert the char back to it ASCII value.
If it fails to convert then it is not a standard ASCII character.
Output Example
PS C:\> ∑ - None ascii character