How to replace multiple regular expression matches in NSString?

Question

I would like to know how is it possible to replace specific characters of an NSString based on some regular expression ([A-G]#?). More analytically, I have a NSString full of chords and spaces
e.g. " C A A# B".

What I want is to change these chords based on a dictionary that I made:
@"A", @"A#",
@"A#", @"B",
@"B", @"C",
@"C", @"C#",
@"C#", @"D",
@"D", @"D#",
@"D#", @"E",
@"E", @"F",
@"F", @"F#",
@"F#", @"G",
@"G", @"G#",
@"G#", @"A",

meaning that
A -> A# and A#-> B.
I would also like to keep anything that exists after my basic chord, meaning:
Am -> A#m.

I have already successfully scanned the string and replaced the elements, with the ones I wanted, in a new string, however I can't figure out how can I do that while maintaining the spaces of my initial string.

So to sum things, basically I want this:
" C A A# B"
to become this:
" C# A# B C"

Thank you!

This might help: https://developer.apple.com/library/mac/#documentation/Foundation/Reference/NSRegularExpression_Class/Reference/Reference.html — Ramy Al Zuhouri, Jan 10 '13 at 20:40

Mike Brant · Answer 1 · 2013-01-10T20:50:22.000

2

I really don't see the benefit of regex here at all considering you are going to be replacing all items in the source string with varying values. Your best bet might be to simply parse the string into an array:

i.e.

NSArray *stringArray = [myString componentsSeparatedByString:@" "];

Then perform the mapping/replacement on each value in the array using whatever logic you chose (i.e. enumerator with hashtable).

And finally just join the elements back together:

NSString *joinedString = [mappedStringArray componentsJoinedByString:@" "];

edited Jan 10 '13 at 20:50

answered Jan 10 '13 at 20:43

Mike Brant

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How would that handle the requirement "I would also like to keep anything that exists after my basic chord, meaning: Am -> A#m"? Doesn't that mean that you have to test each component whether it *starts* with a string that has to be replaced? I that case I do not see the advantage over using a regex. – Martin R Jan 11 '13 at 06:30

Martin R · Accepted Answer · 2013-01-11T08:00:27.440

The following code replaces all chords from the dictionary while preserving everything else (so that "Am" is replaced by "A#m" as requested):

NSDictionary *transposeDict = @{
    @"A": @"A#", @"A#": @"B", @"B": @"C", @"C": @"C#", @"C#": @"D",
    @"D": @"D#", @"D#": @"E", @"E": @"F", @"F": @"F#", @"F#": @"G",
    @"G": @"G#", @"G#": @"A"
};

NSString *melody = @" C A A# B   Am";
NSMutableString *transposedMelody = [melody mutableCopy];

NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"([A-G]#?)"
                                                                       options:0
                                                                         error:NULL];

NSArray *matches = [regex matchesInString:melody options:0 range:NSMakeRange(0, [melody length])];

for (NSTextCheckingResult *match in [matches reverseObjectEnumerator]) {
    NSString *oldChord = [melody substringWithRange:match.range];
    NSString *newChord = transposeDict[oldChord];
    if (newChord != nil)
        [transposedMelody replaceCharactersInRange:match.range withString:newChord];
}


NSLog(@"old: %@", melody);
NSLog(@"new: %@", transposedMelody);

Output:

old:  C A A# B   Am
new:  C# A# B C   A#m

The array matches contains all ranges of matching substrings. These ranges are then processed in reverse order (from last match to first match), so that mutating the string (which might change the length) does not influence the locations of the remaining ranges.

Thank you so much! It works perfectly. Your for loop with reverseObjectEnumerator is brilliant! I was just looping with an index and then appending. Thanks again! — user1644365, Jan 11 '13 at 12:26

How to replace multiple regular expression matches in NSString?

2 Answers2