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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Amazon Chime 4.0.5528 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
Sparkle 2.1.0 - $79.99
Sparkle will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
Bookends 12.7.9 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Dash 4.0.0 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
TrailRunner 3.8.834 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
Dash 4.0.0 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Bookends 12.7.9 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
TrailRunner 3.8.834 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
iFFmpeg 6.2.5 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
DaisyDisk 4.4 - $9.99
DaisyDisk allows you to visualize your disk usage and free up disk space by quickly finding and deleting big unused files. The program scans your disk and displays its content as a sector diagram... Read more

RPG Djinn Caster (Games)
RPG Djinn Caster 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: SPECIAL PRICE 38% OFF(USD 7.99 -> USD 4.99)!!!A Fantasy Action RPG of far foreign lands! Summon the Djinns and rise to... | Read more »
Alto's Odyssey gets its first trail...
There's finally video evidence of Alto's Odyssey, the follow up to the 2015 App Store hit, Alto's Adventure. It looks just as soothing and atmospheric as Alto's last outing, but this time players will be journeying to the desert. Whereas Alto's... | Read more »
Last week on Pocket Gamer
What’s going on in the wider world of portable gaming? Each week we ask that question of our sister website Pocket Gamer. The PG team covers iOS gaming, just like 148Apps, but it also strays into the world of Android games and handheld consoles... | Read more »
Pokémon GO Generation 2 evolution guide
At long last, Niantic Labs finally unleashed the Generation 2 Pokémon into the wild. Pokémon GO trainers are scrambling to grab up this new set of 80 Pokémon. There are some special new tricks required to catch all of these new beasties, though.... | Read more »
The best new games we played this week
It feels as though the New Year got off to a creaking start as far as mobile games go, but that's changed over the past few weeks. The last few days alone have seen the debut of a number of wonderful games, so we thought we'd take the time to... | Read more »
Recruit more scallywags and discover new...
Get ready to show off your sea legs all over again in Oceans & Empires’ new grand update, which aims to make the act of rising to the role of seven seas ruler even more fresh and appealing, thanks to a richness of new content on both iOS and... | Read more »
Mage the Ascension: Refuge (Games)
Mage the Ascension: Refuge 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: The groundbreaking roleplaying game Mage: The Ascension manifests in our turbulent present with Refuge, an... | Read more »
Vampire: Prelude (Games)
Vampire: Prelude 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: The classic roleplaying game Vampire: The Masquerade returns to digital games with a Prelude of things to come. Experience a... | Read more »
Digby Forever Guide: How to dig to the d...
Digby Forever is a sparkling homage to arcade classics, and while you may be tiring of the number of arcade games being thrown at you, this endless digger finds many ways to stand out from the rest of the pack. The game manages to be challenging... | Read more »
The best sales on the App Store this wee...
It's been quite the week in mobile games, but if the latest releases(there were some pretty darn good ones, in case you missed out) aren't really doing the trick, perhaps some of these discounted games will. Many of these premium games had their... | Read more »

Price Scanner via MacPrices.net

Apple Certified Refurbished iMacs available f...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Apple offering Certified Refurbished Series 1...
Apple is now offering Certified Refurbished Series 1 and Series 2 Apple Watches for 14-16% off MSRP, starting at $229. An Apple one-year warranty is included with each watch. Shipping is free: Series... Read more
1.4GHz Mac mini on sale for $449, save $50
B&H Photo has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP Read more
12-inch Retina MacBooks on sale for $200-$250...
Newegg has the 12″ 1.2GHz Space Gray Retina MacBook (sku MLH82LL/A) on sale for $1349.99 including free shipping. Their price is $250 off MSRP, and it’s the lowest price available for this model.... Read more
13-inch 2.0GHz Space Gray MacBook Pro on sale...
Adorama has the non-Touch Bar 13″ 2.0GHz Space Gray MacBook Pro in stock today for $100 off MSRP. Shipping is free, and Adorama charges NY & NJ sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray... Read more
13-inch Touch Bar MacBook Pros on sale for $1...
B&H Photo has select 2016 Apple 13″ Touch Bar MacBook Pros in stock today and up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar MacBook... Read more
KSI-1802R SX Disinfect-able Keyboard With Wav...
KSI has unveiled a new, innovative medical keyboard, the KSI-1802R SX, at HIMSS 2017, running February 19-22 in Orlando, Florida. KSI-1802R SX is the only keyboard that combines dual factor... Read more
15-inch Touch Bar MacBook Pros on sale for up...
Amazon has 15″ Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP including free shipping: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray: $2720.38 $79 off MSRP - 15″ 2.7GHz... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
13-inch 1.6GHz/256GB MacBook Air on sale for...
Newegg has the 13″ 1.6GHz/256GB MacBook Air (MMGG2LL/A) on sale for $1029.99 including free shipping. Their price is $170 off MSRP, and it’s the lowest price available for this model. Choose Newegg... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
*Apple* Technician - nfrastructure (United S...
Let’s Work Together Apple Technician This position is based in Portland, ME Life at nfrastructure At nfrastructure, we understand that our success results from our Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.