.net regex balancing groups countenance - relating balanced

conduct explode. i've got twine i'm perplexing match:

other things blah blah....
                    {
                        things stuff
                        {key:
                            stuff
                            stuff
                        }
                    } some-more stuff.....

here's regex:

[^{}]*                      # anything isn't { }
\{                          # starting {
(?>                         # atomic group: pitch divided backtracks exit
    [^{}]+
  |
    \{(?:\w+:)?(?<depth>)   # relating { and discretionary "word:" pull smoke-stack  
  |
    \}(?<-depth>)           # relating } cocktail stack
)*
(?(depth)(?!))              # abyss unbalanced, destroy match
\}                          # finale }
[^{}]*                      # anything isn't { }

so, i'm perplexing review balancing curly braces, where opening curly braces have an discretionary followed colon. above regex matches instance string, nonetheless i mislay curly brace, (i.e. "unbalancing it):

    things blah blah....
                    {
                        things stuff
                        {key:
                            stuff
                            stuff

                    } some-more stuff.....

...it still matches!

can anyone tell me repair regex?