Example.m

- (void) setExpirationTextField:(UITextField*)expirationTextField {
  _expirationTextField = expirationTextField;
  JCTextFieldSetKeyboardToDigits(expirationTextField);
}

- (void) setCardNumberTextField:(UITextField*)cardNumberTextField {
  _cardNumberTextField = cardNumberTextField;
  JCTextFieldSetKeyboardToDigits(cardNumberTextField);
}

- (BOOL) textField:(UITextField*)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString*)replacementString {
  if (textField == self.expirationTextField) {\
    NSArray* digitSets = JCTextFieldHandleCharactersChangeForDigitSetsField(textField, range, replacementString, @[@(2), @(4)], @" / ");

    if (!inTextFieldUpdateRecursion) {
      inTextFieldUpdateRecursion = YES;
      self.creditCard.expirationMonth = digitSets[0];
      self.creditCard.expirationYear = digitSets[1];
      inTextFieldUpdateRecursion = NO;
    }

    return NO;

  } else if (textField == self.cardNumberTextField) {
    __block NSArray* digitSetsLengths = nil;
    [self.cardNumberSectionDigitCountsByCardNumberPrefix enumerateKeysAndObjectsUsingBlock:^(NSString* prefix, NSArray* _digitSetLengths, BOOL* stop) {

      if ( (range.location == 0 && [[textField.text stringByReplacingCharactersInRange:range withString:replacementString] hasPrefix:prefix])
           || (range.location != 0 && [textField.text hasPrefix:prefix])) {
        digitSetsLengths = _digitSetLengths;
        *stop = YES;
      }
    }];
    if (!digitSetsLengths) {
      digitSetsLengths = @[@(4), @(4), @(4), @(4)];
    }

    NSArray* digitSets = JCTextFieldHandleCharactersChangeForDigitSetsField(textField, range, replacementString, digitSetsLengths, @" ");

    if (!inTextFieldUpdateRecursion) {
      inTextFieldUpdateRecursion = YES;
      self.creditCard.cardNumber = [[digitSets componentsJoinedByString:@""] stringByReplacingOccurrencesOfString:@"_" withString:@""];
      inTextFieldUpdateRecursion = NO;
    }

    return NO;
  } else {
    return YES;
  }
}

JCTextFieldFunctions.h

#import <Foundation/Foundation.h>

#ifndef GlobalDriver2Passenger_JCTextFieldFunctions_h
#define GlobalDriver2Passenger_JCTextFieldFunctions_h

NSArray* JCTextFieldHandleCharactersChangeForDigitSetsField(UITextField* textField, NSRange range, NSString* replacementString, NSArray* digitSetLengths, NSString* separator);
void JCTextFieldSetKeyboardToDigits(UITextField* textField);

#endif

JCTextFieldFunctions.m

#import "JCTextFieldFunctions.h"

@implementation NSString (OFStringHelpers)

- (NSString*) stringByRemovingNonDigitsAndUnderscores {
  NSMutableString* string = [[NSMutableString alloc] initWithCapacity:self.length];

  size_t bufferSize = 32;
  NSRange range = NSMakeRange(0, bufferSize);
  NSUInteger end = [self length];
  while (range.location < end) {
    unichar buffer[bufferSize];
    unichar substringBuffer[bufferSize];
    NSUInteger substringLength = 0;
    if (range.location + range.length > end) {
      range.length = end - range.location;
    }
    [self getCharacters:buffer range:range];
    range.location += bufferSize;
    for (NSUInteger i = 0 ; i < range.length; ++i) {
      unichar c = buffer[i];
      if ((c >= '0' && c <= '9') || c == '_') {
        substringBuffer[substringLength] = c;
        ++substringLength;
      }
    }
    [string appendString:[NSString stringWithCharacters:substringBuffer length:substringLength]];
  }

  return string;
}

@end

NSArray* JCTextFieldHandleCharactersChangeForDigitSetsField(UITextField* textField, NSRange range, NSString* replacementString, NSArray* digitSetLengths, NSString* separator) {
  NSString* existingText = (textField.text ? textField.text : @"");
  NSString* textWithReplacements = [existingText stringByReplacingCharactersInRange:range withString:replacementString];

  NSString* filteredExistingText = [existingText stringByRemovingNonDigitsAndUnderscores];
  NSString* filterdTextWithReplacements = [textWithReplacements stringByRemovingNonDigitsAndUnderscores];
  NSString* filteredReplacementString = [replacementString stringByRemovingNonDigitsAndUnderscores];
  NSRange filteredRange = NSMakeRange([[existingText substringToIndex:range.location] stringByRemovingNonDigitsAndUnderscores].length,
                                      [[existingText substringWithRange:range] stringByRemovingNonDigitsAndUnderscores].length);
  if (filteredRange.length < filteredReplacementString.length) {
    // Net addition of characters; overwrite any available underscores.
    NSUInteger filteredRangeEnd = filteredRange.location + filteredRange.length;
    NSUInteger availableUnderscores = 0;
    if (filteredExistingText.length > filteredRangeEnd) {
      availableUnderscores = [filteredExistingText rangeOfCharacterFromSet:[NSCharacterSet characterSetWithCharactersInString:@"_"]
                                                             options:NSAnchoredSearch
                                                               range:NSMakeRange(filteredRangeEnd, filteredExistingText.length - filteredRangeEnd)].length;

    }
    if (availableUnderscores) {
      NSUInteger replacedUnderscores = MIN(availableUnderscores, filteredReplacementString.length - filteredRange.length);
      filterdTextWithReplacements = [filteredExistingText stringByReplacingCharactersInRange:NSMakeRange(filteredRange.location, filteredRange.length + replacedUnderscores)
                                                            withString:filteredReplacementString];
    }
  } else if (filteredRange.length > filteredReplacementString.length) {
    // Net subtraction of characters; replace removed characters with underscores.
    NSUInteger neededUnderscores = filteredRange.length - filteredReplacementString.length;
    NSMutableString* underscores = [[NSMutableString alloc] initWithCapacity:neededUnderscores];
    for (NSUInteger i = 0; i < neededUnderscores; ++i) {
      [underscores appendFormat:@"_"];
    }
    filterdTextWithReplacements = [filteredExistingText stringByReplacingCharactersInRange:NSMakeRange(filteredRange.location, filteredRange.length)
                                                                                withString:[filteredReplacementString stringByAppendingString:underscores]];
  }

  NSUInteger expectedDigitCount = 0;
  for (NSNumber* digitSetLength in digitSetLengths) {
    expectedDigitCount += [digitSetLength unsignedIntegerValue];
  }
  NSUInteger expectedStringLength = expectedDigitCount + ((digitSetLengths.count - 1) * separator.length);

  NSMutableString* paddedFilterdTextWithReplacements = [NSMutableString stringWithString:filterdTextWithReplacements];
  if (expectedDigitCount > filterdTextWithReplacements.length) {
    for (NSUInteger i = 0, len = expectedDigitCount - filterdTextWithReplacements.length; i < len; ++i) {
      [paddedFilterdTextWithReplacements appendString:@"_"];
    }
  }

  NSUInteger index = range.location + replacementString.length;
  NSString* textWithReplacementsToIndex = [textWithReplacements substringToIndex:index];
  NSUInteger filteredIndex = index - (textWithReplacementsToIndex.length - [textWithReplacementsToIndex stringByRemovingNonDigitsAndUnderscores].length);

  NSMutableArray* digitSets = [[NSMutableArray alloc] init];
  NSUInteger separatorsBeforeIndex = 0;
  BOOL filteredIndexIsAtSeparatorBoundary = NO;
  NSUInteger currentDigitSetsDigitStringIndex = 0;
  for (NSNumber* _digitSetLength in digitSetLengths) {
    NSUInteger digitSetLength = [_digitSetLength unsignedIntegerValue];
    [digitSets addObject:[paddedFilterdTextWithReplacements substringWithRange:NSMakeRange(currentDigitSetsDigitStringIndex, digitSetLength)]];
    currentDigitSetsDigitStringIndex += digitSetLength;

    if (filteredIndex > currentDigitSetsDigitStringIndex) {
      ++separatorsBeforeIndex;
    } else if (filteredIndex == currentDigitSetsDigitStringIndex) {
      filteredIndexIsAtSeparatorBoundary = YES;
    }
  }

  if (filteredIndexIsAtSeparatorBoundary && filteredReplacementString.length > filteredRange.length) {
    ++separatorsBeforeIndex;
  }

  NSString* text = [digitSets componentsJoinedByString:separator];
  textField.text = text;

  NSUInteger calcuatedCursorIndex = (separatorsBeforeIndex * separator.length) + filteredIndex;
  NSUInteger cursorIndex = MIN(calcuatedCursorIndex, expectedStringLength);

  UITextPosition* start = [textField positionFromPosition:[textField beginningOfDocument]
                                                   offset:cursorIndex];
  UITextPosition* end = [textField positionFromPosition:start
                                                 offset:0];
  [textField setSelectedTextRange:[textField textRangeFromPosition:start toPosition:end]];

  return digitSets;
}

void JCTextFieldSetKeyboardToDigits(UITextField* textField) {
  if ([UIDevice currentDevice].userInterfaceIdiom == UIUserInterfaceIdiomPad) {
    textField.keyboardType = UIKeyboardTypeNumbersAndPunctuation;
  } else {
    textField.keyboardType = UIKeyboardTypeDecimalPad;
  }
}