How to get first four letters from string in Swift 3?

I am trying to do IFSCCode validation in swift, but the problem I'm facing is I am not able to fetch the first four letters in string.

IFSC Code example:

ABCD0200000

This is how an IFSC Code looks:

1. First four characters in IFSC Code are always alphabets
2. Fifth character is always a zero.
3. And rest can be anything
4. The length of IFSC Code should not be greater than or less than 11. It should 11 in length.

I have written code for ifs code validation in Objective C, but I am not that familiar in Swift so getting a problem in replicating the same in Swift.

The following is the code which I have written in Objective C:

- (BOOL)validateIFSCCode:(NSString*)ifsccode {
  if (ifsccode.length < 4) {
    return FALSE;
  }
  for (int i = 0; i < 4; i++) {
    if (![[NSCharacterSet letterCharacterSet]
            characterIsMember:[ifsccode characterAtIndex:i]]) {
      return FALSE;
    }
  }
  if (ifsccode.length < 10) {
    return FALSE;
  }
  if ([ifsccode characterAtIndex:4] != '0') {
    return FALSE;
  } else {
    return TRUE;
  }
}

In Swift 3

func validateIfscCode(_ ifscCode : String) -> Bool{
    if(ifscCode.characters.count < 4){
        return false;
    }

    for( _ in 0 ..< 4){
        let charEntered = (ifscCode as NSString).character(at: i)
    }
    if(ifscCode.characters.count < 10){
        return false;
    }

    let idx = ifscCode[ifscCode.index(ifscCode.startIndex, offsetBy: 4)]

    print("idx:%d",idx)
    if (idx == "0"){
    }
    return true
}

Solution 1:

The easiest way to get a string's prefix is with

// Swift 3
String(string.characters.prefix(4))

// Swift 4 and up
string.prefix(4)

// Objective-C
NSString *firstFour = [string substringToIndex:4];

Solution 2:

This is a simple validation using regular expression. The pattern represents:

• ^ must be the beginning of the string
• [A-Za-z]{4} = four characters A-Z or a-z
• 0one zero
• .{6} six arbitrary characters
• $ must be the end of the string

Updated to Swift 4

func validateIFSC(code : String) -> Bool {
  let regex = try! NSRegularExpression(pattern: "^[A-Za-z]{4}0.{6}$")
  return regex.numberOfMatches(in: code, range: NSRange(code.startIndex..., in: code)) == 1
}

PS: To answer your question, you get the first 4 characters in Swift 3 with

let first4 = code.substring(to:code.index(code.startIndex, offsetBy: 4))

and in Swift 4 simply

let first4 = String(code.prefix(4))