balanced brackets

Regex:

(?x)\[                      # First '['
        (?:
        [^\[\]]+            # Match everything that's not "[]"
        |
        (?<open> \[ )       # Match '[', and capture into 'open'
        |
        (?<-open> \] )      # Match ']', and delete the 'open' capture
        )+
        (?(open)(?!))       # Fails if 'open' stack isn't empty!
    \]                      # Last opening ']'


Tested with:

possible-text [posibility-1/posibility-2] possible-text [[un/]certain/[un/]sure] some-text [I am/I'm] [[un/]certain/[/un]sure]