TweetFollow Us on Twitter

Adding Regular Expressions To Your Cocoa Application.

Volume Number: 19 (2003)
Issue Number: 4
Column Tag: Cocoa Development

Adding Regular Expressions To Your Cocoa Application.

Using MOKit to add the ability to match regular expressions in Cocoa.

by Ron Davis

Does your application need to parse data out of a bunch of text, or match strings that can vary some, but have a regular syntax? Do you have a Find command in your text editor? If you do you need to add regular expression matching to your app. Regular Expressions are textual representations of strings match pattern. They go beyond just finding a string and let you do things like find a string that begins and ends with certain characters, but can have anything in the middle. Or a string that contain four numbers followed by a letter.

I've been around the Mac a long time and never really thought about grep or regex or other commands that use regular expressions. But OSX changes that. Every UNIX geek out there knows about grep and it various offspring. Scripting languages like Perl use regular expressions as well, so I thought I needed to learn about them. Once I did I was hooked, and wanted to use them in my own applications. That lead me to Mike Ferris' MOKit, a Cocoa framework that lets you easily deal with regular expressions in your application.

Introduction to Regular Expressions

We'll start with a quick look at regular expression syntax for those of you who have no idea what I'm talking about. The introduction will be fast and shallow. If you need more information check out the URL in the Bibliography at the end of the article.

  
Symbol         Meaning                       Example
character      The character typed,          A is a, b is b, etc.
               with the exception of 
               special characters.

[character -   Any of a range of .           [a-d] = a,b,c, or d.
 character]    characters
 
.              Period matches any one 
               character, except line 
               breaks.
               
#              Matches any digit.            0,1,2,3,4,5,6,7,8,9 
   
\r             return

\t             tab 

\              The escape character like     \. matches a period. 
               in printf. Putting a slash    \\ matches a slash.
               in front of a special 
               character allows that 
               character to be matched.
               
?              0 or 1 of the previous .      ca?t, matches cat, or ct,
               characters                    but not caat.

*              0 or more of the              ca*t, matches ct, cat, 
               previous characters           caat, caaat.
               
+              1 or more of the              ca+t, matches cat, caat,
               previous characters           caaat, but not ct.
               
^              any character but the         (^r23) any character 
               ones after the carat.         but r, 2, or 3.
   
pattern |      match pattern or pattern.     ca|t, matches ca or t, 
pattern                                      but not cat.

(pattern)      Matching: treats what is      (ca)*t, matches cat, or 
               in the parenthesis as a       cacat, but not ct.
               single character.   c(*?)t,   on string coat, 
               Searching: delineates the     returns "oa".
               information to be 
               remembered in a find.

The last pattern there gives you a hint that regular expression can be used in two different ways. One way is matching, where you have a string and you want to know if it is equal to a regular expression. This returns a Boolean value, either the string matches or it doesn't. The other way to use regular expressions is to find a substring or strings in a longer string. When you do this you give an expression and you specify what part of the matched string you want back by placing that part in parentheses.

Let's look at an example or two. Say you let the user input a seven digit zip code and you want to make sure they didn't put any letters in there. You could get their input string and compare it against the regular expression "#+", which matches 1 or more digits, but wouldn't match an empty string, nor one with letters in it.

Now say you have an HTML tag for a link like <A HREF=http://www.radproductions.net/>RAD productions</A> and you wanted to pull out the URL. You could search with the regular expression "=(.*?)>" and you would get back http://www.radproductions.net. You may wonder why the ? is there. If you just put ".*", which means match 0 or more characters, you get to the end of the string because quotes and brackets are characters too. This is called a greedy search. Putting the ? tells it to only search until it finds the next part of the expression string.

MOKit

MOKit is a Cocoa framework written by Mike Ferris. It contains some text manipulation classes, one of which handles regular expressions. The underlying regular expression engine is actually a standard package written by Henry Spencer and used in one form or another by a lot of interesting things such as tcl and perl. MOKit classes are "not public domain, but they are free" according to the web page. The code can be downloaded at http://www.lorax.com/FreeStuff/MOKit.html. You can get both compiled frameworks and the source to MOKit. Version 2.6 was used for this article.

MOKit has two main parts, classes for text completion and classes for regular expressions. We'll only be talking about the regular expression classes here. These classes are MORegularExpression and MORegexFormatter. MORegularExpression is the main class for handling the evaluation of regular expressions. It is the one we'll use in our sample code. Here's its declaration.

Listing 1: MORegularExpression interface.

@interface MORegularExpression : NSObject <NSCopying, NSCoding> {
  @private
    NSString *_expressionString;
    NSString *_lastMatch;
    NSRange _lastSubexpressionRanges 
                           [MO_REGEXP_MAX_SUBEXPRESSIONS];
    void *_compiledExpression;
    BOOL _ignoreCase;
}
+ (BOOL)validExpressionString:(NSString *)expressionString;
+ (id)regularExpressionWithString:(NSString *)
               expressionString ignoreCase:(BOOL)ignoreCaseFlag;
+ (id)regularExpressionWithString:(NSString *)
               expressionString;
- (id)initWithExpressionString:(NSString *)expressionString
                ignoreCase:(BOOL)ignoreCaseFlag;
    
- (id)initWithExpressionString:(NSString *)
               expressionString;
- (NSString *)expressionString;
- (BOOL)matchesString:(NSString *)candidate;
- (NSRange)rangeForSubexpressionAtIndex:(unsigned)index
                inString:(NSString *)candidate;
- (NSString *)substringForSubexpressionAtIndex:
               (unsigned)index inString:(NSString *)candidate;
- (NSArray *)subexpressionsForString:(NSString *)candidate;
@end

As you can see, it is a fairly simple class. To use a regular expression in your code you create an instance of this class. If you need to keep it around, using the initWithExpressionString methods will probably be easiest. If you're just going to use it in the scope of a single method, use the class methods regularExpressionWithString, so you won't have to deal with releasing. Both of these methods have twins that take an ignoreCase parameter which, if set to YES, will cause evaluations to ignore the case of the characters in the expression and the search string. If you don't explicitly set case sensitivity then searches are case sensitive. Here's an example of how to create an expression for finding HREFs in a string of HTML:

MORegularExpression*   linkURLExp = [MORegularExpression regularExpressionWithString: 
                                    @"<A HREF=.*?</A>" ignoreCase:YES];

If you want to make sure the expression you create is valid you can call the class method validExpressionString, which will return YES if the expression is a valid regular expression. If you want to know what an MORegularExpression object's expression is you can get it from the expressionString accessor.

Now we can actually do some evaluations. As I said previously, there are two ways to use regular expressions, to match a string and to find a sub-string. If you have a string and you want to make sure it conforms to the regular expression you created, you can pass it into matchesString and the result will tell you if it matches. This is what MORegexFormatter does. It is a formatter you can add to a field and it will validate the value in that field by the regular expression you give it.

Getting sub-expressions is interesting. If you just want to find the location in the target string of a sub-string, you can use the rangeForSubexpressionAtIndex method. If you want the whole sub-string back as a new NSString* you use the substringForSubexpressionAtIndex, passing the string you are searching for in the inString parameter. The index is which value in parentheses you want back. There can be 0 to 20 sets of parentheses in a MOKit expression, and the index indicates which one you want the range for. So you could create an expression like "<A HREF=(.*?)>(.*?)</A>" to search for a link in an HTML page. If we used the HTML in Listing 2, and you asked for index 0 you would get the whole HREF tag: "<A HREF=http://www.radproductions.net/>R.A.D. Productions</A>". If you asked for index 1, you'd get the link back "http://www.radproductions.net/". If you asked for index 2, you'd get back the text "R.A.D. Productions".

Listing 2: Sample HTML

<HTML>
<TITLE>R.A.D. Productions Home Page</TITLE>
<BODY>
<A HREF=http://www.radproductions.net/>R.A.D. Productions</A>
</BODY>
</HTML>

In a nutshell, that is all there is to finding sub-strings with MORegularExpression. The last method in the interface, subexpressionsForString, is there for backwards compatibility and I'm not even going to explain it.

There is one tricky thing about using MORegularExpression in a large amount of text. What happens if you want to find every link in an HTML page? substringForSubexpressionAtIndex is only going to return the first occurrence in the string. Turns out there is no way to say, start searching at character n in the candidate string. What I did was truncate the string after each search to find the next one. Here's my code to find all of the links and their URL in an HTML page.

Listing 3: Finding all of the links.

-(void)handleHTML:(NSString*)inHTML
{
   MORegularExpression*   bothExp = 
                        [MORegularExpression 
                        regularExpressionWithString:
                        @"<A HREF=(.*?)>(.*?)</A>" 
                        ignoreCase:YES];
   
   MORegularExpression*   startStopExp = 
                        [MORegularExpression 
                        regularExpressionWithString:
                        @"<HTML>(.*?)</HTML>"];
   NSString*            result = nil;
   NSRange               range;
   NSString*            curString = [startStopExp 
                        substringForSubexpressionAtIndex:1
                        inString:inHTML];
   
   do 
   {
      range = [bothExp rangeForSubexpressionAtIndex:0
                     inString:curString ];
      if ( range.length > 0 )
         {
         NSString*   URLString;
         NSString*   linkString;
         NSURL*      fullURL;
         
         result = [linkURLExp 
                        substringForSubexpressionAtIndex:0
                        inString:curString];
         URLString = [bothExp 
                        substringForSubexpressionAtIndex:1
                        inString:curString];
         fullURL = [NSURL URLWithString:URLString 
                        relativeToURL:baseURL];
         URLString = [fullURL absoluteString];
         
         linkString = [bothExp
                        substringForSubexpressionAtIndex:2
                        inString:curString];
         if ( linkString == nil || 
               URLString == nil || 
               ([linkString length]== 0) || 
                     ([URLString length]== 0) )
            {} else 
            {
            [self addURL:URLString withText:linkString];
            }
         curString = [curString substringFromIndex:
                        (range.location + range.length)];
         }
   }
   while ([curString length] > 0 && 
               range.location != NSNotFound );
}

A little explanation. The method is in a class that has a method addURL. The class also keeps two arrays, one for URLs and one for the link text. When you call addURL the URL and the link string are added to the arrays for future reference. The class also knows what the URL of the page you are parsing is, and saves it in a variable called baseURL.

The first thing the method does is set up our regular expression for links. Then it makes a new string that will contain only the text between the <HTML> tag. You can use this to limit the search to just a certain part of the page. Then it sets up a loop, which will always execute once and will end when we don't get anything back from our search, or we run out of HTML to parse. Inside the loop we first try to find our expression's range in the HTML. If it isn't there, were done. If we find something, then we use our expression to get the sub-string for the URL. Some times a URL will be relative, so we use NSURL with the page's URL to create a full URL. Then we ask for the second index, which is the link text. If we get both, we add it to our list.

If we find something, then we need to search from the end of the string we found. So we create a sub-string from our current HTML string, that starts at the end of what we found and ends at the end of the current string. This effectively chops off everything from the beginning of the string to the end of what we just found. Then we loop.

Hopefully you've seen the coolness of regular expressions and want to use them in your Cocoa apps. MOKit makes this easy and is easy to use. So go to Mike Ferris' website and download it and add regular expressions to your app.

Bibliography

Mastering Regular Expressions, Jeffrey E. F. Friedl,

http://www.ora.com/catalog/regex2/

Using Regular Expressions, Stephen Ramsay,

http://etext.lib.virginia.edu/helpsheets/regex.html

Regular Expressions specification,

http://www.opengroup.org/onlinepubs/007908799/xbd/re.html

A Tao of Regular Expressions, http://sitescooper.org/tao_regexps.html

BBEdit Grep Tutorial, http://www.anybrowser.org/bbedit/grep.shtml


Ron Davis is a long time Mac programmer, having worked on everything from Virex Anti-Virus to CodeWarrior. His day job is working for Alsoft, and his evening job is R.A.D. Productions, makers of Suck It Down and FinderEye.

 
AAPL
$102.13
Apple Inc.
+1.24
MSFT
$44.87
Microsoft Corpora
-0.14
GOOG
$571.00
Google Inc.
-6.86

MacTech Search:
Community Search:

Software Updates via MacUpdate

Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
BBEdit 10.5.12 - Powerful text and HTML...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Microsoft Office 2011 14.4.4 - Popular p...
Microsoft Office 2011 helps you create professional documents and presentations. And since Office for Mac 2011 is compatible with Office for Windows, you can work on documents with virtually anyone... Read more
TextWrangler 4.5.10 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more
BitTorrent Sync 1.4.72 - Sync files secu...
BitTorrent Sync allows you to sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything. File transfers are encrypted. Your information... Read more
Cyberduck 4.5.2 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Tinderbox 6.0.3 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more

Latest Forum Discussions

See All

Mobile Convolution (Music)
Mobile Convolution 1.0.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0.0 (iTunes) Description: | Read more »
Invaders! From Outer Space Review
Invaders! From Outer Space Review By Rob Thomas on August 27th, 2014 Our Rating: :: RETRO NOSTALGIAUniversal App - Designed for iPhone and iPad It’s a shame that Invaders! doesn’t offer deeper gameplay, as this retro-inspired... | Read more »
Spooklands Review
Spooklands Review By Jennifer Allen on August 27th, 2014 Our Rating: :: ONE-TOUCH SHOOTERUniversal App - Designed for iPhone and iPad One-touch simultaneously controls your direction and your weapon in this unique arena shooter.   | Read more »
Heroes of Order & Chaos Add Twitch I...
Heroes of Order & Chaos Add Twitch Integration, New Heroes, and More Posted by Ellis Spice on August 27th, 2014 [ permalink ] | Read more »
Foodie Yama Review
Foodie Yama Review By Jennifer Allen on August 27th, 2014 Our Rating: :: BRIEFLY HOOKSUniversal App - Designed for iPhone and iPad Foodie Yama will draw you in for a brief while, and you’ll never be entirely sure why.   | Read more »
Spotify Connect Turns One, Now Supports...
Spotify Connect Turns One, Now Supports New Devices Posted by Ellis Spice on August 27th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Rise of PicsaStock and How You Can M...
We all take plenty of photos, right? That’s the joy of having a reasonably powerful camera in your pocket, thanks to your trusty iPhone and a bevy of similarly useful apps. Wouldn’t it be great to make some money out of those snaps? While your... | Read more »
Appointment With F.E.A.R Review
Appointment With F.E.A.R Review By Jennifer Allen on August 27th, 2014 Our Rating: :: CAMP DELIGHTUniversal App - Designed for iPhone and iPad Ever wanted to be a superhero? Appointment With F.E.A.R is a fine way to live that dream... | Read more »
Indie MOBA ÆRENA’s 2.0 Summer Update Bri...
Indie MOBA ÆRENA’s 2.0 Summer Update Brings New Missions, Skins, and a Champion Posted by Jessica Fisher on August 27th, 2014 [ permalink | Read more »
Ultra Drift Review
Ultra Drift Review By Rob Thomas on August 27th, 2014 Our Rating: :: ULTRA DULLUniversal App - Designed for iPhone and iPad Ultra Drift is fine in concept, but the execution runs head-first into the wall. And so will you. Over and... | Read more »

Price Scanner via MacPrices.net

The iPad’s Real Competitive Challenger (Not S...
It’s been my contention for some time that the iPad is suffering from something of an identity crisis, and I suspect that may be a factor in slackening sales this year. Apple can’t seem to decide... Read more
13-inch 2.6GHz/256GB Retina MacBook Pro on sa...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Life Inventory iOS Apps – Learn to Know Thyse...
James Hollender’s Life Inventory apps s are now on sale with 20% off thru Labor Day, 09/01/2014. This is a great opportunity to get started on that Moral Inventory you’ve been putting off doing for... Read more
Pocket Watch, LLC. Reveals Cloud Server For P...
Beaumont, Texas based Pocket Watch, LLC. has announced the availability of its new ActivePrint Cloud Server Powered by Raspberry Pi. With this small standalone box almost any USB printer or available... Read more
902it Simplifies Area Code Changes For Nova S...
The east coast Canadian provinces of Nova Scotia and Prince Edward Island are phasing in 10 digit telephone dialing, to be fully in place by November, in order to accommodate a second area code to... Read more
Boomerang iPad Stand Mounts Your iPad Anywher...
Boomerang, a Mountable Stand with Multiple Viewing Angles, is now available for iPad Air. Boomerang combines several functions that aim to expand your iPad’s potential in one, elegant product. The... Read more
Retina MacBook Pros available starting at $10...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more
Apple 27-inch Thunderbolt Display (refurbishe...
The Apple Store has Apple Certified Refurbished 27″ Thunderbolt Displays available for $799 including free shipping. That’s $200 off the cost of new models. Read more
Apple offers free $25 iTunes gift card with p...
The Apple Store is offering a free $25 iTunes Gift Card with the purchase of a $99 Apple TV for a limited time. Shipping is free. Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.