I'm struggling a bit trying to figure out what regular expression to use for search & replace

For example, I want to replace all occurrences of text that begins with [MSG_74_ and ends with ] and add a quote " after the [ and before the ]

i.e.

msg[MSG_74_HELLO]

to

msg["MSG_74_HELLO"]

A break down of what each bit of the regex expression does would help too.