Regex nested / recursive pattern

I am trying to decode an encoded string k[encoded_string] to k * encoded_string. So, I am looking at using regex to accomplish this.
How to construct a regex pattern which has nested / recursive matches.

For example,

regex re("(\\d+)(\\[[a-zA-Z]+\\])");
str = "3[a]2[bc]";

This should match 3[a] and 2[bc] and it does:

Found: 2 matches
Match: 3[a]
  Submatch 0: 3[a]
  Submatch 1: 3
  Submatch 2: [a]
Match: 2[bc]
  Submatch 0: 2[bc]
  Submatch 1: 2
  Submatch 2: [bc]

However, for a string `”3[a2[c]]”`, I expect it to match:

1. 2[c]
2. 3[a decoded (1) ]

However, the regex I tried only manages to capture 1 match:

Found: 1 matches
Match: 2[c]
  Submatch 0: 2[c]
  Submatch 1: 2
  Submatch 2: [c]

It misses the `3a[]` outer match which contains the 2[c] shown above. This is tricky.