Objective-C implementation of mod97 IBAN checking algorithm

NSString+Mod97Check.h

#import <Foundation/Foundation.h>

@interface NSString (Mod97Check)

- (BOOL)passesMod97Check; // Returns result of mod 97 checking algorithm. Might be used to check IBAN.
                          // Expects string to contain digits and/or upper-/lowercase letters; space and all the rest symbols are not acceptable.

@end

NSString+Mod97Check.m

#import "NSString+Mod97Check.h"

@implementation NSString (Mod97Check)

- (BOOL)passesMod97Check {
    NSString *string = [self uppercaseString];
    NSInteger mod = 0, length = [self length];
    for (NSInteger index = 4; index < length + 4; index ++) {
        unichar character = [string characterAtIndex:index % length];
        if (character >= '0' && character <= '9') {
            mod = (10 * mod + (character - '0')) % 97; // '0'=>0, '1'=>1, ..., '9'=>9
        }
        else if (character >= 'A' && character <= 'Z') {
            mod = (100 * mod + (character - 'A' + 10)) % 97; // 'A'=>10, 'B'=>11, ..., 'Z'=>35
        }
        else {
            return NO;
        }
    }
    return (mod == 1);
}

@end

This is a real-life implementation

static NSString* const Letters = @"ABCDEFGHIJKLKMNOPQRSTUVWXYZ";
static NSString* const Decimals = @"0123456789";
static NSString* const LettersAndDecimals = @"ABCDEFGHIJKLKMNOPQRSTUVWXYZ0123456789";

+(BOOL)isValidIBAN:(NSString *)iban
{
    iban = [[iban stringByReplacingOccurrencesOfString:@" " withString:@""] uppercaseString];
    NSCharacterSet *invalidChars = [[NSCharacterSet characterSetWithCharactersInString:LettersAndDecimals] invertedSet];

    if ([iban rangeOfCharacterFromSet:invalidChars].location != NSNotFound)
    {
        return NO;
    }

    int checkDigit = [iban substringWithRange:NSMakeRange(2, 2)].intValue;
    iban = [NSString stringWithFormat:@"%@%@",[iban substringWithRange:NSMakeRange(4, iban.length - 4)], [iban substringWithRange:NSMakeRange(0, 4)]] ;

    for (int i = 0; i < iban.length; i++) {
        unichar c = [iban characterAtIndex:i];
        if (c >= 'A' && c <= 'Z') {
            iban = [NSString stringWithFormat:@"%@%d%@", [iban substringWithRange:NSMakeRange(0, i)], (c - 'A' + 10),[iban substringWithRange:NSMakeRange(i+1, iban.length - i - 1)]];
        }

    }
    iban = [[iban substringWithRange:NSMakeRange(0, iban.length - 2)] stringByAppendingString:@"00"];

    while(true)
    {
        int iMin = (int)MIN(iban.length, 9);
        NSString* strPart = [iban substringWithRange:NSMakeRange(0, iMin)];
        int decnumber = strPart.intValue;
        if(decnumber < 97 || iban.length < 3)
            break;
        int del = decnumber % 97;
        iban =  [NSString stringWithFormat:@"%d%@", del, [iban substringFromIndex:iMin]];
    }
    int check = 98 - iban.intValue;

    return checkDigit == check;
}