Replace Delimited Tokens in a String

Problem Statement

You are given a template string where tokens are enclosed in curly braces (e.g., {key}). You are also provided with a dictionary mapping keys to replacement values. Write a function that replaces all occurrences of {key} in the string with its corresponding value from the dictionary.

Rules:

1. If a key is found in the dictionary, replace {key} with the value.
2. If a key is not found, leave the {key} placeholder as-is in the final string.
3. If a placeholder is malformed (e.g., {key without a closing brace, or {} empty braces), leave the string segment unchanged.
4. You do not need to handle nested placeholders (e.g., {a{b}}).

Function Signature

def replace_tokens(template: str, replacements: dict[str, str]) -> str:

Examples

Example 1:

• Input: template = "Hello {name}, welcome to {place}!", replacements = {"name": "Alice", "place": "Amazon"}
• Output: "Hello Alice, welcome to Amazon!"

Example 2:

• Input: template = "Invalid {token and {empty} are ignored", replacements = {"empty": "space"}
• Output: "Invalid {token and space are ignored"

Constraints

• template length is between 0 and 10,000 characters.
• The dictionary can contain up to 1,000 key-value pairs.
• Time complexity should be linear with respect to the length of the string.

Edge Cases

• Empty template string.
• Dictionary missing keys.
• Consecutive braces or incomplete braces.