FastfileParser.swift

let descExpression = try! NSRegularExpression(pattern: "^\\s*desc [\"']{1}([^\"]*)[\"']{1}", options: [])
// …
let description = (0..<lineIndex)
  .reversed()
  .prefixMap({ index -> String? in
    // Get a line
    let line = lines[index]
    // Use regular expression
    guard let match = descExpression.firstMatch(in: line) else { return nil }
    // Regular expression contains a capture group, therefore looking for 2 ranges
    guard match.numberOfRanges >= 2 else { return nil }
    // When there are matches then extract description
    guard let range = Range(match.range(at: 1), in: line) else { return nil }
    return String(line[range])
  })
  .reversed()
  .joined(separator: "\n")