Formatting UILabel contents in US phone number format

Question

Note: As I've posted this question, it's night time here in my place. If I haven't responded to comments or answers, I'll do it in the morning. Thank you for your understanding :-)

The idea is to format UILabel as phone number (US Format) as the user touches each UIButton.

There is a keypad layout and as the user touches each numeric key, the label displays it and at formats the resulting string on the fly. I have written a piece of code which I am not very proud of. I hope someone could help me in doing this in much better way or different approach.

The image should give you a fair idea of how it looks like. Now, getting to the logic:

In the code below, keyOne is the IBAction to append the numbers to UILabel, keyBack is the one to delete it.

So this is how it works for entering a phone number:

If I enter 1234567890:

after 1,2,3 the label is modified as 123- and gets assigned to string
Now string will be 123- and I continue touching 4567890

after 123-4567, when I touch 8, label is modified as (123)456-78 and gets assigned to string
Now string will be (123)456-7890

If I wish, I could continue entering more numbers. 
As per requirement, the number should lose its formatting. 
i.e. After (123)456-7890 if I press 1, it should become 12345678901

Now, the trick is to undo the steps in exact same way. Here is the pickle. If I entered only 1234567890, _phoneNumber.text would be (123)456-7890 and pressing back three times should result in 123-4567. Then pressing back four times should result in 123

However, if I entered 12345678901, _phoneNumber.text would go to (123)456-7890, then lose formatting to become 12345678901 and when I press back once, it should become (123)456-7890 and so on.

If you whip up a new project of type single view application and paste the stuff below, (of course after creating buttons and connecting 0-9 with keyOne, back with keyBack) you would see that it works fine. But as I mentioned above, I have written a piece of code which I am not very proud of. I feel there should be a simpler way to do this.

ViewController.h

@interface ViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *lengthParam;
@property (weak, nonatomic) IBOutlet UILabel *phoneNumber;
- (IBAction)keyOne:(id)sender;
- (IBAction)keyBack:(id)sender;
- (IBAction)clearAll:(id)sender;
@end

ViewController.m

#import "ViewController.h"

@interface ViewController () {
    BOOL isReformatted;
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

- (IBAction)keyOne:(id)sender {
    UIButton *btn = (UIButton *)sender;
    NSString *str = [NSString stringWithString:self.phoneNumber.text];
    str = [str stringByAppendingString:btn.titleLabel.text];
    self.phoneNumber.text = str;
    if(str.length==4) self.phoneNumber.text = [self insertMinus:str]; //before the 4th number, insert '-' as a first step
    if(str.length==9) self.phoneNumber.text = [self insertParenthesis:str]; //remove - before the 4th number, encapsulate first 3 numbers inside () and add - at end of current number
    if(str.length>13) self.phoneNumber.text = [self plainFormat:str]; //if user enter more than 10 numbers, remove formatting
    self.lengthParam.text = [NSString stringWithFormat:@"%d",self.phoneNumber.text.length];
}

- (IBAction)keyBack:(id)sender {
    NSString *str = [NSString stringWithString:self.phoneNumber.text];
    NSString *newStr = nil;
    if(str.length>0) { //check for empty string
        if(str.length>11 && isReformatted==NO) newStr = [str substringToIndex:str.length-1]; //string.length > 10 which means plainFormat was called earlier and reFormat isn't called yet
        else if(str.length==11 && isReformatted == NO) { //string length is now 11
            if([str characterAtIndex:0]!='(') newStr = [self reFormat:str]; //if entered string is 12345678901, it is not reFormatted. remove last 1 and reFormat it as (123)456-7890
            else newStr = [self removeParenthesis:str]; //entered string itself is (123)456-78 so transform it as 123-4567
        } else { //we are dealing with a reformatted string of length 11 now.
            newStr = [str substringToIndex:str.length-1]; //String is (123)456-78, remove 8 and apply one of the below rules
            if(newStr.length==10) { //transform (123)456-7 as 123-4567
                newStr = [str substringToIndex:str.length-1];
                newStr = [self removeParenthesis:str];
            }
            if(newStr.length==4&&[newStr characterAtIndex:3]=='-') newStr = [self removeMinus:str]; //transform 123-4 to 123
        }
        self.phoneNumber.text = newStr;
        self.lengthParam.text = [NSString stringWithFormat:@"%d",self.phoneNumber.text.length];
    }
}

- (IBAction)clearAll:(id)sender {
    self.phoneNumber.text = @"";
    self.lengthParam.text = [NSString stringWithFormat:@"%d",self.phoneNumber.text.length];
}

- (NSString *)insertMinus:(NSString *)str {
    return [NSString stringWithFormat:@"%@-%@",[str substringToIndex:3],[str substringFromIndex:3]];
}

- (NSString *)insertParenthesis:(NSString *)str {
    NSString *c1 = [NSString stringWithFormat:@"(%@)",[str substringToIndex:3]];
    NSString *c2 = [NSString stringWithFormat:@"%@-%@",[str substringWithRange:NSMakeRange(4, 3)],[str substringFromIndex:7]];
    return [NSString stringWithFormat:@"%@%@",c1,c2];
}

- (NSString *)plainFormat:(NSString *)str {
    isReformatted = NO;
    str = [str stringByReplacingOccurrencesOfString:@"(" withString:@""];
    str = [str stringByReplacingOccurrencesOfString:@")" withString:@""];
    str = [str stringByReplacingOccurrencesOfString:@"-" withString:@""];
    str = [str stringByReplacingOccurrencesOfString:@" " withString:@""];
    return str;
}

- (NSString *)reFormat:(NSString *)str {
    isReformatted = YES;
    NSString *c1 = [NSString stringWithFormat:@"(%@)",[str substringToIndex:3]];
    NSString *c2 = [NSString stringWithFormat:@"%@-%@",[str substringWithRange:NSMakeRange(3, 3)],[str substringWithRange:NSMakeRange(6, 4)]];
    return [NSString stringWithFormat:@"%@%@",c1,c2];
}

- (NSString *)removeParenthesis:(NSString *)str {
    str = [self plainFormat:str];
    NSString *newStr = [NSString stringWithFormat:@"%@-%@",[str substringToIndex:3],[str substringWithRange:NSMakeRange(3, 4)]];
    return newStr;
}

- (NSString *)removeMinus:(NSString *)str {
    str = [self plainFormat:str];
    NSString *newStr = [NSString stringWithFormat:@"%@",[str substringToIndex:3]];
    return newStr;
}

@end

Answer 1

If you are eager to go down the road of rolling your own phone number formatter, I would consider doing the follow:

1. Store the user input without formatting in a string.
2. After each key press, reformat the display string.

Something like:

@property (nonatomic, copy) NSMutableString *backingString;
@property (weak, nonatomic) IBOutlet UILabel *phoneNumber;

- (void)viewWillAppear:(BOOL)animated
{
   [super viewWillAppear:animated];
   backingString = [NSMutableString string];
}

- (IBAction)keyOne:(id)sender
{
    UIButton *btn = (UIButton *)sender;
    [self.backingString appendString:btn.titleLabel.text];
    [self formatBackingString];
}


- (IBAction)keyBack:(id)sender
{
    [self.backingString deleteCharactersInRange:(NSRange){self.backingString.length - 1,1}];
    [self formatBackingString];
}

- (void)formatBackingString
{
    // Do your phone formatting here
    NSMutableString *formattedString = [NSMutableString string];
    NSInteger length = self.backingString.length;
    if (length < 3)
    {
       [formattedString appendString:self.backingString];
    }
    else if (length == 3)
    {
       [formattedString appendFormat:@"%@-", self.backingString];
    }
    else if (length < 8)
    {
        // etc...
    }
    self.phoneNumber.text = formattedString;
}

You could also look at NSNumberFormatter.