Separator2.swift

extension String {
    
    /// Returns a new string with the camel-case-based words of this string
    /// split by the specified separator.
    ///
    /// Examples:
    ///
    ///     "myProperty".convertedToSnakeCase()
    ///     // my_property
    ///     "myURLProperty".convertedToSnakeCase()
    ///     // my_url_property
    ///     "myURLProperty".convertedToSnakeCase(separator: "-")
    ///     // my-url-property
    func convertedToSnakeCase(separator: Character = "_") -> String {
        guard !isEmpty else { return self }
        
        return zip(self, indices)
            .map({ (character: Character, idx: Index) -> String in
                let lower = String(character).lowercased()
                
                // Character is not uppercase or is start of string
                if idx == startIndex || !character.isUppercase { return lower }
                
                // Idx is not startIndex
                let previousIdx = index(before: idx)
                
                // An uppercase follows a non-uppercase/non-separator like P in myProperty, insert separator
                if previousIdx != startIndex && !self[previousIdx].isUppercase && self[previousIdx] != separator {
                    return String(separator) + lower
                }
                
                let nextIdx = index(after: idx)
                
                // Final letter _or_ next is uppercase, like L in myURL or R in myURLProperty, do not separate
                if nextIdx == endIndex || self[nextIdx].isUppercase {
                    return lower
                }
                
                // Next is _not_ uppercase, previous _is_ uppercase, like P in myURLProperty. Insert separator
                if self[previousIdx].isUppercase {
                    return String(separator) + lower
                }
                
                return lower
            }).reduce("", +)
    }
}

Separator3.swift

extension String {
    
    /// Returns a new string with the camel-case-based words of this string
    /// split by the specified separator.
    ///
    /// Examples:
    ///
    ///     "myProperty".convertedToSnakeCase()
    ///     // my_property
    ///     "myURLProperty".convertedToSnakeCase()
    ///     // my_url_property
    ///     "myURLProperty".convertedToSnakeCase(separator: "-")
    ///     // my-url-property
    func convertedToSnakeCase(separator: Character = "_") -> String {
        guard !isEmpty else { return self }
        
        var result = ""
        
        for (character, idx) in zip(self, indices) {
            let lower = String(character).lowercased()
            
            // Character is not uppercase or is start of string
            if idx == startIndex || !character.isUppercase {
                result += lower
                continue
            }
            
            // An uppercase follows a non-uppercase/non-separator like P in myProperty, insert separator
            if !self[index(before: idx)].isUppercase &&
                self[index(before: idx)] != separator {
                result += String(separator) + lower
                continue
            }
            
            let nextIdx = index(after: idx)
            
            // Final letter _or_ next is uppercase, like L in myURL or R in myURLProperty, do not separate
            if nextIdx == endIndex || self[nextIdx].isUppercase {
                result += lower
                continue
            }
            
            // Next is _not_ uppercase, previous _is_ uppercase, like P in myURLProperty. Insert separator
            if  self[index(before: idx)].isUppercase {
                result += String(separator) + lower
                continue
            }
            
            result += lower
        }
        
        return result
    }
}

print("m_P_r_operty".convertedToSnakeCase(),
      "myProperty".convertedToSnakeCase(),
      "myURLProperty".convertedToSnakeCase(),
      "myURLProperty".convertedToSnakeCase(separator: "-"))

Separator_Original.swift

extension String {
  /// Returns a new string with the camel-case-based words of this string
  /// split by the specified separator.
  ///
  /// Examples:
  ///
  ///     "myProperty".convertedToSnakeCase()
  ///     // my_property
  ///     "myURLProperty".convertedToSnakeCase()
  ///     // my_url_property
  ///     "myURLProperty".convertedToSnakeCase(separator: "-")
  ///     // my-url-property
  func convertedToSnakeCase(separator: Character = "_") -> String {
    guard !isEmpty else { return self }
    var result = ""
    // Whether we should append a separator when we see a uppercase character.
    var separateOnUppercase = true
    for index in indices {
      let nextIndex = self.index(after: index)
      let character = self[index]
      if character.isUppercase {
        if separateOnUppercase && !result.isEmpty {
          // Append the separator.
          result += "\(separator)"
        }
        // If the next character is uppercase and the next-next character is lowercase, like "L" in "URLSession", we should separate words.
        separateOnUppercase = nextIndex < endIndex && self[nextIndex].isUppercase && self.index(after: nextIndex) < endIndex && self[self.index(after: nextIndex)].isLowercase
      } else {
        // If the character is `separator`, we do not want to append another separator when we see the next uppercase character.
        separateOnUppercase = character != separator
      }
      // Append the lowercased character.
      result += character.lowercased()
    }
    return result
  }
  
}