GSM 0338 encoding for SMS

Posted by on Aug 27, 2012 in iOS, Mac OS X, Mobile Applications | One Comment

Here’s a little Objective-C helper class we wrote to make it easier to convert NSString’s to GSM 03.38 encoding on the iPhone, iPad, or Mac:

  • Handles stripping out characters not included in the GSM0338 character set
  • Calculates the length of a GSM 0338 string (e.g. ^,[,|,etc each require 14 bits instead of 7
  • Gives you a truncate method (e.g. ^hello truncated to length 4 is ^he because ^ is 14 bits)
#import <Foundation/Foundation.h>

@interface GSM0338Helper : NSObject
+(int)gsmStringLength:(NSString *)textToShare;
+(NSString *)gsmStringFromString:(NSString *)textToShare replacementChar:(NSString *)replace;
+(NSString *)gsmShorten:(NSString *)text length:(int)length truncationText:(NSString *)truncationText;
#import "GSM0338Helper.h"

@implementation GSM0338Helper

// gsm has certain characters like pipe that require 14 bits instead of 7 so we need to double count those
+(int)gsmStringLength:(NSString *)textToShare {
    if (textToShare == nil || [textToShare length] == 0) return 0;
    NSError *error = NULL;
    // count the number of times special escape characters appear that will take up 2 characters instead of one
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"[\\^\\{\\}\\[~\\]\\|€\\\\]" options:NSRegularExpressionCaseInsensitive error:&error];
    NSArray *matches = [regex matchesInString:textToShare options:NSRegularExpressionCaseInsensitive range:NSMakeRange(0, [textToShare length])];
    return [textToShare length] + [matches count];

// replaces non gsm characters with the replace string you specify, can be empty string @""
+(NSString *)gsmStringFromString:(NSString *)textToShare replacementChar:(NSString *)replace {
    if (textToShare == nil || [textToShare length] == 0) return textToShare;
    NSError *error = NULL;
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"[^A-Za-z0-9@£$¥èéùìòÇ\fØø\nÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ !\"#¤%%&'()\\*\\+,-./:;<=>\\?¡ÄÖÑܧ¿äöñüà\\^\\{\\}\\[~\\]\\|€\\\\]" options:NSRegularExpressionCaseInsensitive error:&error];
    NSString *modifiedString = [regex stringByReplacingMatchesInString:textToShare options:0 range:NSMakeRange(0, [textToShare length]) withTemplate:replace];
    return modifiedString;

// shortens gsm string to length gsm chars and if it truncates it, it appends the truncation text
+(NSString *)gsmShorten:(NSString *)text length:(int)length truncationText:(NSString *)truncationText {
    if (textToShare == nil || [textToShare length] == 0) return text;
    int gsmTextLength = [self gsmStringLength:text];
    if (gsmTextLength > length) {
        // We have to first figure out how many characters to remove since gsm length is possibly > [title length]
        int numCharsToRemove = gsmTextLength - length;
        int newTextLength = [text length] - numCharsToRemove;
        if (newTextLength > 0 && newTextLength < [text length])
            text = [NSString stringWithFormat:@"%@%@", [text substringToIndex:newTextLength], truncationText];
    return text;


1 Comment

  1. Namal Jayatilaka
    September 11, 2012

    Please I need some help to clarify about a SMS.
    After sent a 365 characters long message (including all letters, digits, spaces and symbols) and encoded by GSM service provider, with in the encoded SMS can it be show as 383 characters long ???


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