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Regular Expressions for NSString

Last weekend I was looking for different options to validate NSString objects against some regular expressions.
While there are some 3rd party classes for doing regular expressions in Cocoa and wrappers for Perl/PCRE there is a way to do that without external software. The magic word is called [NSPredicate](http://developer.apple.com/documentation/Cocoa/Reference/Foundation/Classes/NSPredicate_Class/Reference/NSPredicate.html).
This approach may perhaps not be obvious because there is (at this time) no direct documentation or hint in the [ADC documentation](http://developer.apple.com).

NSPredicate was introduced in OS X 10.4 and is mainly used in CoreData applications to specify queries and has therefore also some nice functionality to filter arrays.
But there is also a way to use its built-in regular expression mechanism to validate NSStrings.

Let's say you have a string "Hello World!" and you want to check if there are at least two sequent l's in it. The regular expression would be: ".\*l{2,}.\*" (without quotes).
Note, that there is a dot followed by an asterisk (which means "zero or more characters") at the beginning and at the end of the actual expression (l{2,}). This is necessary because (unlike the regular expressions in Perl) the "interpolated length" of the regular expression must be at least the same as the length of the string to test to (I don't know why but that's what I found out during my tests).

In your Cocoa application this test would look like this:

NSString *mystring = @"Hello World!";
NSString *regex = @".*l{2,}.*";

NSPredicate *regextest = [NSPredicate
predicateWithFormat:@"SELF MATCHES %@", regex];

if ([regextest evaluateWithObject:mystring] == YES) {
NSLog(@"Match!");
} else {
NSLog(@"No match!");
}

This is easy, isn't it?
All you have to do is to create a NSPredicate object with the predicate "SELF MATCHES" followed by a regular expression and call its "evaluateWithObject:" method with your string you want to validate. It returns YES if the string matches your regular expression and NO if it does not.

If you don't like this approach or if you don't want your application to require OS X 10.4 you can find some other solutions and documentation regarding regular expressions in Cocoa at [this webpage](http://www.cocoadev.com/index.pl?RegularExpressions).