Provides Keyword-formatting to the String class

KeywordFormatterPlayground.swift

String(format: "The value of life is %(life)$d", ["life":42])
// "The value of life is 42"

String(format: "¿Dónde está %(firstname)$@ %(lastname)$@?", ["firstname":"Carmen", "lastname":"San Diego"])
// "¿Dónde está Carmen San Diego?"

String(format: "PI is %(pi)$.4f", ["pi":Double.pi])
// "PI is 3.1416"

String(format: "But PI can also be %(pi)$.f, or %(pi)$.5f, depending how you format it", ["pi":Double.pi])
// "But PI can also be 3, or 3.14159, depending how you format it"

String(format: "😇😍%(emoji)$@", ["emoji":"😁"])
// "😇😍😁"

String+KeywordFormatter.swift

import Foundation

public extension String
{
    // Look for %(word)$ and replace it
    private static let __keywordRegexp = try! NSRegularExpression(pattern: "%\\((.+?)\\)\\$", options: [])
    
    public init(format text: String, _ keywords: [String : CVarArg])
    {
        // Matches for %(value)$, capturing
        let range  = NSRange(location: 0, length: (text as NSString).length)
        
        // Results
        var keys : [String] = []
        
        // Save all the captures
        for match in String.__keywordRegexp.matches(in: text, options: [], range: range)
        {
            keys.append((text as NSString).substring(with: match.rangeAt(1)))
        }
        
        // Create the final format string
        let format = String.__keywordRegexp.stringByReplacingMatches(in: text, options: [], range: range, withTemplate: "%")
        
        self.init(format: format, arguments: keys.map { keywords[$0] }.flatMap { $0 })
    }
}