trim.swift

extension String {
   
    func trimBackTo(boundary: Character) -> String {
        if isEmpty {return ""}
        if self[endIndex.predecessor()] == boundary { return "" }
        return characters.split(boundary).map(String.init).last!
    }
    
    func trimUpTo(boundary: Character) -> String {
        if isEmpty { return "" }
        if self[startIndex] == boundary {return String(self[startIndex])}
        return characters.split(boundary, maxSplit: 1, allowEmptySlices: false).map(String.init).first!
    }
    
}

extension String {
    /// Retake on "lastPathComponent" and "pathExtension"
    /// but a little more general in behavior
    ///
    /// - Author: aciidb
    func snipBackTo(boundary: Character) -> String {
        if isEmpty || self[endIndex.predecessor()] == boundary { return "" }
        return characters.split(boundary).map(String.init).last ?? self
    }

    /// Snip forward to boundary
    ///
    /// - Author: aciidb
    func snipUpTo(boundary: Character) -> String {
        if isEmpty || self[startIndex] == boundary {return ""}
        return characters.split(boundary, maxSplit: 1, allowEmptySlices: false).map(String.init).first ?? self
    }


    /// Trim back from end
    func trimBackTo(boundary: Character) -> String {
        if isEmpty {return ""}
        if self[endIndex.predecessor()] == boundary {return self[startIndex...endIndex.predecessor().predecessor()]}
        let last = characters.split(boundary).map(String.init).last ?? ""
        if last == self {return self}
        return String(characters.dropLast(last.characters.count + 1)) ?? self
    }

    /// Trim forward to boundary
    func trimUpTo(boundary: Character) -> String {
        if isEmpty { return "" }
        if self[startIndex] == boundary { return self[startIndex.successor()...endIndex.predecessor()] }
        let first = characters.split(boundary).map(String.init).first ?? ""
        if first == self {return self}
        return String(characters.dropFirst(first.characters.count + 1)) ?? self
    }
}

var path = "/Users/ericasadun/Desktop/feefs.txt/filename.fext"
path.snipBackTo("Z") == path
path.snipBackTo("/") == "filename.fext"
path.snipBackTo(".") == "fext"

path.snipUpTo("Z") == path
path.snipUpTo("/") == ""
path.snipUpTo(".") == "/Users/ericasadun/Desktop/feefs"

path.trimBackTo("Z") == path
path.trimBackTo("/") == "/Users/ericasadun/Desktop/feefs.txt"
path.trimBackTo(".") == "/Users/ericasadun/Desktop/feefs.txt/filename"
path.trimBackTo("t") == "/Users/ericasadun/Desktop/feefs.txt/filename.fex"

path.trimUpTo("Z") == path
path.trimUpTo("/") == "Users/ericasadun/Desktop/feefs.txt/filename.fext"
path.trimUpTo(".") == "txt/filename.fext"

path = "x"
path.snipBackTo("Z") == "x"
path.snipUpTo("Z") == "x"
path.trimUpTo("Z") == "x"
path.trimBackTo("Z") == "x"