ReplacingTokens.swift

let text = "The quick <color_1> <animal_1> jumps over the lazy <animal_2>"
let replacementMap = ["<animal_1>": "fox", "<animal_2>": "dog", "<color_1>": "brown"]

extension String {
    func replacingOccurrences(matchingPattern pattern: String, replacementProvider: (String) -> String?) -> String {
        let expression = try! NSRegularExpression(pattern: pattern, options: [])
        let matches = expression.matches(in: self, options: [], range: NSRange(startIndex..<endIndex, in: self))
        return matches.reversed().reduce(into: self) { (current, result) in
            let range = Range(result.range, in: current)!
            let token = String(current[range])
            guard let replacement = replacementProvider(token) else { return }
            current.replaceSubrange(range, with: replacement)
        }
    }
}

let finalString1 = text.replacingOccurrences(matchingPattern: "<[:alpha:]+_{1}[:digit:]+>", replacementProvider: { replacementMap[$0] })
let finalString2 = text.replacingOccurrences(matchingPattern: "<[:alpha:]+_{1}[:digit:]+>", replacementProvider: { _ in "REPLACEMENT" })
print(text)
print(finalString1) // The quick brown fox jumps over the lazy dog
print(finalString2) // The quick REPLACEMENT REPLACEMENT jumps over the lazy REPLACEMENT

Found your gist helpful - thanks!
Then saw another option with Regex:

let finalString3 = text.replacing(/<[:alpha:]+_{1}[:digit:]+>/) { replacementMap["\($0.0)", default: "\($0.0)"] }