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When developing, we often come across code we’ve seen hundreds of times that we just copy and paste from project to project. Sometimes it’s so banal that it’s not worth trying to improve, change or even modularize it. Ctrl-C + Ctrl-V may be the cheapest and fastest solution, ergo, the best.

This is the kind of situation we see when masks must be applied to text fields such as Social Security Numbers (SSN), Telephones, Zip Codes, and so on. Those awful few dozens of lines we copy from one textField:shouldChangeCharactersInRange:replacementString: to the other. It never changes because it works. There’s no need to improve it. It’s always the same. But I’ve got tired of it. That nasty code that, in a well structured class, can be as a mole in a beautiful woman’s face.

A few days ago I got some spare time and I started working on something I though could come handy. So how do you simplify and generalise this problem that every developer has to face at least once? Easy! Regex! Wait.. You sure it’s easy? I’ll tell you, it wasn’t easy, but we’re talking about patterns in varying occasions and, as I visualised it, regex is the best, simplest and most “approachable” solution. I mean, everybody knows a little bit of regex.

While modelling, I wanted to make it as simple and as clean as possible, therefore, for now, there are only two classes, and only one method you’ll really use, but you can read the complete documentation. From now on, I’ll present a superficial approach to introduce this small library.

Fork the Github Repo!


The NSStringMask is the main class responsible for applying your masks to the NSStrings. Among it’s methods the only noteworthy is [NSStringMask maskString:withPattern:placeholder:].

It formats the given string based on the pattern you provide, filling the voids with the placeholder. It automatically creates an instance of NSRegularExpression with the option CaseInsensitive, so if you need a different option, you may use the alternate method which expects a regex instead of pattern.

There’s also an alternative method that receives no placeholder (same as nil), in this case, when the given string is shorter than expected, instead of applying the mask, the method will return a cleaned string having only the valid characters based on validCharactersForString:.

Important Note

Doesn’t matter if providing a pattern or an instance of regex, the regular expression must have capturing groups (parentheses). This is because the class understands that everything that is not within parentheses is part of the mask and must be interpreted as literal characters!

Suppose you’re formatting a SSN, whose regex pattern is \d{3}-\d{2}-\d{3}. If you provide that regex/pattern, nothing will be formatted because the class will think that \d{3} is part of the mask, and not 3 numbers that should go there.

The correct usage is: (\d{3})-(\d{2})-(\d{3})

NSString *result = [NSStringMask maskString:@"12345678" withPattern:@"(\d{3})-(\d{2})-(\d{3})"];
// result = @"123-45-678"

NSString *result = [NSStringMask maskString:@"12345678" withPattern:@"\d{3}-\d{2}-\d{3}"];
// result = nil

Obs: If Xcode is showing a warning about unknown escape sequences, try adding double slashes (\\d)


This is a subclass of UITextField, so you can set your nibs to use this class and automatically apply masks to your text fields.

Obviously, you must provide a mask to transform your strings, and it’s recommended that you do so when initialising your view or view controller. So you should set the text field’s mask inside viewDidLoad or awakeFromNib.

Suppose you have a view with an outlet to an UITextFieldMask named textFieldMask:

- (void)awakeFromNib
    NSStringMask *mask = [NSStringMask maskWithPattern:@"(\d+)"];

    textFieldMask.mask = mask;
    textFieldMask.delegate = self;

A question for you: what does that mask accepts?


You can install it from Cocoa Pods, or clone the project from the Github Repo and copy the files in the folder Classes.

Helpful stuff

I’ve also created this gist with some common patterns. Feel free to improve it!

Bugreport 101

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