Regex+MapOutput.swift

import RegexBuilder

extension Regex {
    
    /// Transforms the output of a regex component using the provided transformation function.
    ///
    /// - Parameter transform: A closure that takes the output of the regex and transforms it to a new output type.
    /// - Returns: A new `Regex` instance that applies the transformation function to its output.
    public func mapOutput<NewOutput>(_ transform: @escaping (Output) throws -> NewOutput) -> Regex<NewOutput> {
        Regex<NewOutput> {
            MapOutput(innerRegex: self, transform: transform)
        }
    }
}

private struct MapOutput<Output, NewOutput>: CustomConsumingRegexComponent {
    
    typealias RegexOutput = NewOutput
    
    let innerRegex: Regex<Output>
    let transform: (Output) throws -> NewOutput
    
    func consuming(
        _ input: String,
        startingAt index: String.Index,
        in bounds: Range<String.Index>) throws -> (upperBound: String.Index, output: NewOutput)?
    {
        guard index != bounds.upperBound, let match = input[index..<bounds.upperBound].prefixMatch(of: innerRegex) else {
            return nil
        }
        let output = try transform(match.output)
        return (match.range.upperBound, output)
    }
}