String Parsing

In this article

In almost every computer program, we have to parse strings. Sometimes these strings follow a very simple format, and sometimes they are very complicated. We will look at multiple ways to convert these strings into something we can reason and work with. We’ll discuss regular expressions and scanners and parsers, as well as when to apply them.

Regular vs. Context-Free Grammars

First, a little bit of background theory: if we parse a string, we interpret the string in a specific language. For example, if we want to parse the string @"42" to a number, we interpret the string in the language of natural numbers. A grammar is used to describe a language: it is the collection of rules according to which a string can be interpreted. In the case of natural numbers, there can be only one rule: a string can be interpreted, if and only if it is a sequence of digits. This language could be described using some standard C functions, or using a regular expression. If we can describe a language using regular expressions, it is said to have a regular grammar.

If we consider expressions like “1 + 2 * 3”, parsing becomes a bit more difficult. For these expressions, the language can be described using an inductive grammar. In other words, this is a grammar that has rules that refer to themselves, possibly even in a recursive way. For example, to recognize this language, we could have three rules:

  1. Any number is part of the language
  2. If x is a member of the language, and y is a member, then x + y is also a member
  3. If x is a member of the language, and y is a member, then x * y is also a member

The languages that can be described using these kinds of grammars are called context-free grammars, or CFG. Note that they cannot be parsed using regular expressions (although some regular expression implementations, such as PCRE, can express more than regular grammars). The classical example of languages that can be parsed with CFG but not with regular expressions is that of matching parentheses.

Some examples of things that can be parsed using regular languages are numbers, strings, and dates. This means that you can use a regular expression (or a similar technique) to parse them.

Some things that cannot be parsed using regular languages, are e-mail addresses, JSON, XML, or most programming languages. To parse these, we need an actual parser. Often times, these parsers are already written for us. Apple provides parsers for XML and JSON, so if we want to parse those, it’s easiest to just use Apple’s format.

Regular Expressions

When you want to recognize simple languages, regular expressions are often the right tool for the job. However, they are also frequently misused for things like HTML parsing, where they are not a good fit. Let’s suppose we have a file containing simple variable definitions for colors that designers can use to change the colors in your iPhone app. The format looks like this:

backgroundColor = #ff0000

If we want to parse a single line using this format, we can use a regular expression, like below. The pattern is the most important thing. If you don’t know regular expressions, we will quickly go over what this means, but explaining regular expressions completely is way beyond the scope of this article. The first thing to look at is \\w+, which matches a word character (defined by \\w), one or more times (defined by the +). Then, to make sure we can later use the result of that match, we put it in parentheses, creating a capture group. Next, there’s a literal space character, followed by an equals sign, another space character, and a pound. Then, we need to match six hexadecimal numbers. The \\p{Hex_Digit} matches a hexadecimal digit (Hex_Digit is a unicode property name). The modifier {6} means that we expect six of those characters, and again, we put it in parentheses to create a second capture group:

NSError *error = nil;
NSString *pattern = @"(\\w+) = #(\\p{Hex_Digit}{6})";
NSRegularExpression *expression = [NSRegularExpression regularExpressionWithPattern:pattern
                                                                            options:0
                                                                              error:&error];
NSTextCheckingResult *result = [expression firstMatchInString:string 
                                                      options:0
                                                        range:NSMakeRange(0, string.length)];
NSString *key = [string substringWithRange:[result rangeAtIndex:1]];
NSString *value = [string substringWithRange:[result rangeAtIndex:2]];

We create a regular expression and ask the expression to match the string string. Then we extract the two groups captured by the parentheses by using rangeAtIndex. The entire regular expression is at index 0, the first capture group is at index 1, the second capture group is at index 2, and so on. Now key is backgroundColor and value is ff0000. This regular expression parses a single line; a next step would be to parse multiple lines and add some error checking. For example, the input:

backgroundColor = #ff0000
textColor = #0000ff

should produce the following dictionary: @{@"backgroundColor": @"ff0000", @"textColor": @"0000ff"}. The code to do that is simple. We split the string into lines, iterate over them, and add them to our dictionary:

NSString *pattern = @"(\\w+) = #([\\da-f]{6})";
NSRegularExpression *expression = [NSRegularExpression regularExpressionWithPattern:pattern
                                                                            options:0 
                                                                              error:NULL];
NSArray *lines = [input componentsSeparatedByCharactersInSet:[NSCharacterSet newlineCharacterSet]];
NSMutableDictionary *result = [NSMutableDictionary dictionary];
for (NSString *line in lines) {
    NSTextCheckingResult *textCheckingResult = [expression firstMatchInString:line 
                                                                      options:0 
                                                                        range:NSMakeRange(0, line.length)];
    NSString* key = [line substringWithRange:[textCheckingResult rangeAtIndex:1]];
    NSString* value = [line substringWithRange:[textCheckingResult rangeAtIndex:2]];
    result[key] = value;
}
return result;

As an aside: to separate the string into components, we could have also used componentsSeparatedByString: or enumerated them using enumerateSubstringsInRange:options:usingBlock: with the option NSStringEnumerationByLines.

To see if a line doesn’t match (for example, if we accidentally forget one of the hexadecimal characters), we can check if textCheckingResult is nil, and throw an error:

 if (!textCheckingResult) {
     NSString* message = [NSString stringWithFormat:@"Couldn't parse line: %@", line]
     NSDictionary *errorDetail = @{NSLocalizedDescriptionKey: message};
     *error = [NSError errorWithDomain:MyErrorDomain code:FormatError userInfo:errorDetail];
     return nil;
 }

Scanners

There is a second way of taking this string and turning it into a dictionary, namely using scanners. Conveniently, Foundation provides us with NSScanner, which has an easy-to-use object-oriented API. First, we need to create a scanner:

NSScanner *scanner = [NSScanner scannerWithString:string];

By default, a scanner skips all white space and newline characters. For our purposes, we don’t want to skip newlines, only white space:

scanner.charactersToBeSkipped = [NSCharacterSet whitespaceCharacterSet];

Then, we define the set of hexadecimal characters. A lot of character sets are defined, but the hexadecimal set is not one of them:

NSCharacterSet *hexadecimalCharacterSet = 
  [NSCharacterSet characterSetWithCharactersInString:@"0123456789abcdefABCDEF"];

First, let’s write a version without error checking. A scanner works like this: it takes a string and sets its cursor to 0, the beginning of the string. You tell it to scan something specific, like [scanner scanString:@"=" intoString:NULL]. The method returns YES if the scan succeeds, and increases the cursor value to just after the scanned part. The method scanCharactersFromSet:intoString: works in a similar way: it keeps scanning characters from the set, and puts the result into the string pointer given by the second argument. Note that we combine the different scanning calls with &&. This way, the right-hand side of the && is only scanned if the left-hand side succeeded:

NSMutableDictionary *result = [NSMutableDictionary dictionary];
while (!scanner.isAtEnd) {
    NSString *key = nil;
    NSString *value = nil;
    NSCharacterSet *letters = [NSCharacterSet letterCharacterSet];
    BOOL didScan = [scanner scanCharactersFromSet:letters intoString:&key] &&
                   [scanner scanString:@"=" intoString:NULL] &&
                   [scanner scanString:@"#" intoString:NULL] &&
                   [scanner scanCharactersFromSet:hexadecimalCharacterSet intoString:&value] &&
                   value.length == 6;
    result[key] = value;
    [scanner scanCharactersFromSet:[NSCharacterSet newlineCharacterSet] 
                        intoString:NULL]; // scan an optional newline
}
return result;

To add error handling, we can write this code just after the didScan line. If scanning didn’t succeed, we just return nil and set the error parameter accordingly. When parsing text, it’s important to think about what you want to do in case a string is malformed, whether it is to crash, present the error to the user, try to recover from the error, etcetera:

    if (!didScan) {
        NSString *message = [NSString stringWithFormat:@"Couldn't parse: %u", scanner.scanLocation];
        NSDictionary *errorDetail = @{NSLocalizedDescriptionKey: message};
        *error = [NSError errorWithDomain:MyErrorDomain code:FormatError userInfo:errorDetail];
        return nil;
    }

Note that C also provides you with scanner functions, such as man 3 sscanf. They follow a similar syntax to printf, but operate in the inverse order (parsing a string, rather than generating one).

Parsers

What if our designers would also want to specify RGB colors, like this: (100,0,255)? We would have to make our method for parsing colors a bit smarter. As a matter of fact, after we’re done here, we will have written a very basic parser.

First, we will add a couple of more methods to our class, and store our scanner in a property. The first method we add is called scanColor:, and its job is to scan either a hex color (like ff0000) or an RGB tuple (e.g. (255,0,0)):

- (NSDictionary *)parse:(NSString *)string error:(NSError **)error
{
    self.scanner = [NSScanner scannerWithString:string];
    self.scanner.charactersToBeSkipped = [NSCharacterSet whitespaceCharacterSet];

    NSMutableDictionary *result = [NSMutableDictionary dictionary];
    NSCharacterSet *letters = [NSCharacterSet letterCharacterSet]
    while (!self.scanner.isAtEnd) {
        NSString *key = nil;
        UIColor *value = nil;
        BOOL didScan = [self.scanner scanCharactersFromSet:letters intoString:&key] &&
                       [self.scanner scanString:@"=" intoString:NULL] &&
                       [self scanColor:&value];
        result[key] = value;
        [self.scanner scanCharactersFromSet:[NSCharacterSet newlineCharacterSet] 
                                 intoString:NULL]; // scan an optional newline
    }
}

The scanColor: method itself is very easy. First, it tries to scan a hex color, and if that doesn’t work, it tries an RGB tuple:

- (BOOL)scanColor:(UIColor **)out
{
    return [self scanHexColorIntoColor:out] || [self scanTupleColorIntoColor:out];
}

Scanning a hex color is the same as before. The only difference is that we have now wrapped it in a method, and use the same pattern as NSScanner methods. It returns a BOOL indicating successful scanning, and stores the result in a pointer to a UIColor:

- (BOOL)scanHexColorIntoColor:(UIColor **)out
{
    NSCharacterSet *hexadecimalCharacterSet = 
       [NSCharacterSet characterSetWithCharactersInString:@"0123456789abcdefABCDEF"];
    NSString *colorString = NULL;
    if ([self.scanner scanString:@"#" intoString:NULL] &&
        [self.scanner scanCharactersFromSet:hexadecimalCharacterSet 
                                 intoString:&colorString] &&
        colorString.length == 6) {
        *out = [UIColor colorWithHexString:colorString];
        return YES;
    }
    return NO;
}

Scanning a tuple-based color is very similar; we have already looked at all the necessary methods for doing this. We scan tokens like @"(", interspersed by the integer components. In production code, we would need some more error checking, to, for example, make sure that the integers are in the range 0-255:

- (BOOL)scanTupleColorIntoColor:(UIColor **)out
{
    NSInteger red, green, blue = 0;
    BOOL didScan = [self.scanner scanString:@"(" intoString:NULL]
            && [self.scanner scanInteger:&red]
            && [self.scanner scanString:@"," intoString:NULL]
            && [self.scanner scanInteger:&green]
            && [self.scanner scanString:@"," intoString:NULL]
            && [self.scanner scanInteger:&blue]
            && [self.scanner scanString:@")" intoString:NULL];
    if (didScan) {
        *out = [UIColor colorWithRed:(CGFloat)red/255. 
                               green:(CGFloat)green/255.
                                blue:(CGFloat)blue/255. alpha:1];
        return YES;
    } else {
        return NO;
    }
}

When we start to mix scanning things with logic–such as scanning multiple alternatives, and calling other methods–we are writing a parser. Parsers are a fascinating subject, and a very powerful tool in your arsenal. Once you know how to write a parser, you can invent small languages for anything: specifying style sheets, parsing constraints, querying your data model, describing business logic, and so on. An interesting book on this is Fowler’s Domain Specific Languages.

Tokenization

We have a very simple parser that can extract key/value pairs from a string (for example, coming from a file) and we can use those strings to generate UIColor objects. But we’re not done yet. What if the designers want to specify more things? For example, suppose we have a different file that contains some layout constraints, in the following form:

myView.left = otherView.right * 2 + 10
viewController.view.centerX + myConstant <= self.view.centerX

How could we parse this? It turns out that regular expressions are not the best way to do this.

Before we parse this, it’s a good idea to do tokenization. This is the process that converts a string (a stream of characters) into a stream of tokens. For example, the result of scanning myConstant = 100 might be @[@"myConstant", @"=", @100]. For most languages, tokenization removes white spaces and parses related characters into tokens. In our language, tokens could be identifiers (e.g. myConstant or centerX), operators (e.g. ., + or =), or numbers (e.g. 100). After tokenization, the tokens are parsed into their final form.

For tokenization (which is also sometimes called lexing or scanning), we can reuse our NSScanner class. First, we can focus on parsing strings that only contain operators:

NSScanner *scanner = [NSScanner scannerWithString:contents];
NSMutableArray *tokens = [NSMutableArray array];
while (![scanner isAtEnd]) {
  for (NSString *operator in @[@"=", @"+", @"*", @">=", @"<=", @"."]) {
      if ([scanner scanString:operator intoString:NULL]) {
          [tokens addObject:operator];
      }
  }
}

The next step is to also recognize identifiers such as myConstant and viewController. For our simple purposes, identifiers can only consist of letters (no digits). This is how we scan them:

NSString *result = nil;
if ([scanner scanCharactersFromSet:[NSCharacterSet letterCharacterSet] 
                        intoString:&result]) {
    [tokens addObject:result];
}

The method scanCharactersFromSet:intoString: will only return YES if those characters can be found, and then we add it to our tokens array. We’re almost done now; the only thing left is parsing numbers. Luckily, NSScanner has some methods for that too. We can just use scanDouble: to scan doubles, make an NSNumber out of them, and add them to the tokens:

double doubleResult = 0;
if ([scanner scanDouble:&doubleResult]) {
    [tokens addObject:@(doubleResult)];
}

Now our scanner is complete, and we can test it:

NSString* example = @"myConstant = 100\n"
                    @"\nmyView.left = otherView.right * 2 + 10\n"
                    @"viewController.view.centerX + myConstant <= self.view.centerX";
NSArray *result = [self.scanner tokenize:example];
NSArray *expected = @[@"myConstant", @"=", @100, @"myView", @".", @"left", 
                      @"=", @"otherView", @".", @"right", @"*", @2, @"+", 
                      @10, @"viewController", @".", @"view", @".", 
                      @"centerX", @"+", @"myConstant", @"<=", @"self", 
                      @".", @"view", @".", @"centerX"];
XCTAssertEqualObjects(result, expected);

Our scanner creates separate tokens for the operators and names and puts the numbers into NSNumber objects. Once that’s complete, we’re ready to take the second step: parsing the array of tokens into something more meaningful.

Parsing

There’s a reason why we can’t solve this problem with regular expressions or just scanners and need a more powerful technique. It’s because parsing might fail. For example, consider when we see the token @"myConstant". In our parsing function, we don’t know if this is the start of a constraint expression or a constant definition. We will need to try both options and see which succeeds. We can implement this by hand (which is not that hard, but gets messy), or use the right tool for the job: a parsing library.

First, we need to describe our language in a format that the library can understand. Here’s the grammar for our constraint-parsing language. This is written in EBNF:

constraint = expression comparator expression
comparator = "=" | ">=" | "<="
expression = keyPath "." attribute addMultiplier addConstant
keyPath = identifier | identifier "." keyPath
attribute = "left" | "right" | "top" | "bottom" | "leading" | "trailing" | "width" | "height" | "centerX" | "centerY" | "baseline"
addMultiplier = "*" atom
addConstant = "+" atom
atom = number | identifier

There are multiple Objective-C libraries available for parsing (see CocoaPods). One of them is CoreParse, which provides us with an API that works really well in Objective-C. However, we cannot directly feed the grammar above into CoreParse. The library only has parsers with a lookahead of one. This means that whenever the parser has to decide between two rules (for example, in the rule keyPath) it will look at the next token and make the decision. If we find out only later that we should have chosen the other rule, we’re in trouble. There are other parsers which allow for more ambiguous grammars, but this comes at a large performance penalty.

Luckily, we can do some refactoring on our grammar, in order to make sure it’s compatible with the parser library. We can also convert it to Backus-Naur Form, and now it’s exactly in the format that CoreParse expects:

NSString* grammarString = [@[
    @"Atom ::= num@'Number' | ident@'Identifier';",
    @"Constant ::= name@'Identifier' '=' value@<Atom>;",
    @"Relation ::= '=' | '>=' | '<=';",
    @"Attribute ::= 'left' | 'right' | 'top' | 'bottom' | 'leading' | 'trailing' | 'width' | 'height' | 'centerX' | 'centerY' | 'baseline';",
    @"Multiplier ::= '*' num@'Number';",
    @"AddConstant ::= '+' num@'Number';",
    @"KeypathAndAttribute ::= 'Identifier' '.' <AttributeOrRest>;",
    @"AttributeOrRest ::= att@<Attribute> | 'Identifier' '.' <AttributeOrRest>;",
    @"Expression ::= <KeypathAndAttribute> <Multiplier>? <AddConstant>?;",
    @"LayoutConstraint ::= lhs@<Expression> rel@<Relation> rhs@<Expression>;",
    @"Rule ::= <Atom> | <LayoutConstraint>;",
] componentsJoinedByString:@"\n"];

If a rule is matched, then the library tries to find a class with the same name (for example, Expression). If that class implements a method initWithSyntaxTree:, then that method is called. Alternatively, the parser has a delegate, which gets callbacks whenever a rule is matched (or when an error occurs). For example, in the case of the relation rule, we get back a CPSyntaxTree, and the first child of that tree is a keyword token containing either @"=", @">=", or @"<=". We can map that string to an NSNumber containing the layout attribute using a dictionary:

- (id)parser:(CPParser *)parser didProduceSyntaxTree:(CPSyntaxTree *)syntaxTree
    NSString *ruleName = syntaxTree.rule.name;
    if ([ruleName isEqualToString:@"Attribute"]) {
        return self.layoutAttributes[[[syntaxTree childAtIndex:0] keyword]];
    }
    ...

The full code for our parser is on GitHub, and in a class of slightly more than 100 lines we can parse complicated layout constraints like:

viewController.view.centerX + 20 <= self.view.centerX * 0.5

And get a result like below, which can easily be converted into an NSLayoutConstraint:

(<Expression: self.keyPath=(viewController, view), 
              self.attribute=9,
              self.multiplier=1, 
              self.constant=20> 
 -1 
 <Expression: self.keyPath=(self, view), 
              self.attribute=9,
              self.multiplier=0.5,
              self.constant=0>)

Other Tools

Instead of Objective-C libraries, another common strategy is to use tools like Bison, Yacc, Ragel, or Lemon, which are all C-level libraries.

Another thing you could do is use these parsers to generate part of your code at build time. For example, once you have a parser for a language, you can create a simple command-line wrapper around it. Add an Xcode build rule, and you have a compiler for your own language that gets executed on each build.

Parsing Ideas

It might seem like parsing is a bit weird, and creating string-based languages doesn’t feel very Objective-C-like. The opposite is true: Apple is doing this all the time. For example, think about NSLog format strings, NSPredicate strings, the layout constraint visual formatting language, and even key-value coding. All of these use small internal parsers to parse strings into objects and actions. Often you don’t have to write a parser yourself, which will save a lot of work: common languages like JSON and XML are already taken care of. But if you would want to write a calculator, a graphics language, or even an embedded Smalltalk, parsers are your friend.