Which of the following strings ought match:

[SYN]
[syn]
[Syn]
[SYN, ACK]
[SYN] [SYN, ACK]
[ACK]
SYN
SYN, ACK
ACK
[download]

What pattern did you try specifically? The following pattern may be what you want:

/\[SYN(?:, ACK)?\]/
[download]

Update: couple more test strings added