Problem

At AegisCloud, certificate identifiers must follow a strict string format before they can be processed. Implement a function that returns True if a certificate string is valid and False otherwise.

Rules

A certificate string is valid if all of the following are true:

1. It contains exactly three non-empty segments separated by colons: header:payload:signature.
2. Each segment contains only uppercase letters (A-Z), digits (0-9), and the delimiter characters (, ), [, ], {, }.
3. Across the entire string, all delimiters are balanced and properly nested.
4. The string must not start or end with a colon, and it must not contain consecutive colons.

Formal Specification

• Input: cert, a string
• Output: bool — whether the certificate format is valid

Examples

Example 1

• Input: cert = "AB[12]:C{D}3:XYZ9"
• Output: True
• Explanation: There are exactly three non-empty segments, all characters are allowed, and the delimiters are balanced.

Example 2

• Input: cert = "AB[12:C{D}3:XYZ9"
• Output: False
• Explanation: The [ delimiter is never closed.

Constraints

• 1 <= len(cert) <= 10^5
• Input consists of ASCII characters
• The solution should run in linear time