It might not be the optimal way, but it's simple enough for everyone to understand. However, I haven't tested for performance against other methods.

DECLARE @test-2Table(

MyStringvarchar(MAX))

INSERT @test-2 VALUES

('NT AUTHORITY\SYSTEM: [System Admin]YES;[Security Admin];[Server Admin]YES;[setup Admin];[Process Admin];[Disk Admin];[Database Creator]'),

('NT SERVICE\MSSQL$instance1: [System Admin]YES;[Security Admin];[Server Admin];[setup Admin];[Process Admin];[Disk Admin];[Database Creator]'),

('NT SERVICE\SQLAGENT$instance1: [System Admin]YES;[Security Admin];[Server Admin];[setup Admin];[Process Admin];[Disk Admin];[Database Creator]YES')

SELECT LEFT( MyString, CHARINDEX( ':', MyString) + 1) +

CASE WHEN CHARINDEX( 'System Admin]YES;', MyString) > 0

THEN '[System Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'Security Admin]YES;', MyString) > 0

THEN '[Security Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'Server Admin]YES;', MyString) > 0

THEN '[Server Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'setup Admin]YES;', MyString) > 0

THEN '[setup Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'Process Admin]YES;', MyString) > 0

THEN '[Process Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'Disk Admin]YES;', MyString) > 0

THEN '[Disk Admin]YES;' ELSE '' END +

CASE WHEN CHARINDEX( 'Database Creator]YES', MyString) > 0

THEN '[Database Creator]YES;' ELSE '' END

FROM @test-2