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

Arq 5.7.2 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
Spotify 1.0.47.13. - Stream music, creat...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
Together 3.7.3 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
Cocktail 10.2 - General maintenance and...
Cocktail is a general purpose utility for macOS 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
Sierra Cache Cleaner 11.0.3 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
NeoFinder 7.0.1 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Sketch 42 - Design app for UX/UI designe...
Sketch is an innovative and fresh look at vector drawing for the Mac. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus,... Read more
Quicken 4.4.3 - Complete personal financ...
Quicken makes managing your money easier than ever. Whether paying bills, upgrading from Windows, enjoying more reliable downloads, or getting expert product help, Quicken's new and improved features... Read more
Apple Safari 10.0.3 - Apple's Web b...
Note: The direct download link is currently unavailable. It is available in the OS X 10.11.6 release, as well as in the Apple Security Updates. Apple Safari is Apple's web browser that comes with OS... Read more
Apple iTunes 12.5.5 - Play Apple Music a...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more

Clash Royale gets some serious balance u...
| Read more »
Ironhide Game Studio prepares for a busy...
Kingdom Rush breathed fresh life into the tired tower defense genre way back in 2012. The game was a robust challenge that somehow managed to lift you up, rather than leaving you feeling crushed and hopeless. The rich array of unit types and... | Read more »
Collect pets and sling arrows in Arcane...
Mobile gaming is a crowded market, but regular updates are a good way to keep us attention-short players keen. The brand new content in Arcane Online is a prime example. Published by Japanese developer Gala, Arcane Online is a fantasy MMO that... | Read more »
Super Mario Run dashes onto Android in M...
Super Mario Run was one of the biggest mobile launches in 2016 before it was met with a lukewarm response by many. While the game itself plays a treat, it's pretty hard to swallow the steep price for the full game. With that said, Android users... | Read more »
WarFriends Beginner's Guide: How to...
Chillingo's new game, WarFriends, is finally available world wide, and so far it's a refreshing change from common mobile game trends. The game's a mix of tower defense, third person shooter, and collectible card game. There's a lot to unpack here... | Read more »
Super Gridland (Entertainment)
Super Gridland 1.0 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: Match. Build. Survive. "exquisitely tuned" - Rock Paper Shotgun No in-app purches, and no ads! | Read more »
Red's Kingdom (Games)
Red's Kingdom 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Mad King Mac has kidnapped your father and stolen your golden nut! Solve puzzles and battle goons as you explore and battle your... | Read more »
Turbo League Guide: How to tame the cont...
| Read more »
Fire Emblem: Heroes coming to Google Pla...
Nintendo gave us our first look at Fire Emblem: Heroes, the upcoming mobile Fire Emblem game the company hinted at last year. Revealed at the Fire Emblem Direct event held today, the game will condense the series' tactical RPG combat into bite-... | Read more »
ReSlice (Music)
ReSlice 1.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0 (iTunes) Description: Audio Slice Machine Slice your audio samples with ReSlice and create flexible musical atoms which can be triggered by MIDI notes or... | Read more »

Price Scanner via MacPrices.net

HDRtist NX 1.0 For macOS Next Generation HDR...
Ohanaware Co., Ltd. have announced the immediate release of their latest High Dynamic Range Imaging software “HDRtist NX”. HDRtist NX is the third installment in the HDRtist series of applications... Read more
Built To Last Maverick Leather Laptop Messeng...
Wild West melds with modern in San Francisco designer-manufacturer of custom laptop sleeves, bags and cases WaterField Designs’ new Maverick Leather Laptop Messenger. This luxury laptop bag blends... Read more
Logitech Sets Company Record With 15 GOOD DES...
Logitech has announced that it has won 15 GOOD DESIGN 2016 awards. This marks the sixth consecutive year Logitech products have been recognized by the GOOD DESIGN awards, and the most award wins for... Read more
13-inch Touch Bar MacBook Pros in stock for $...
B&H Photo has the new 2016 13″ Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar... Read more
9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9.7″ 32GB Space Gray Apple iPad Pro on sale for $549 for a limited time. Shipping is free, and B&H charges NY sales tax only. Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon has 2015 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) on sale for $1749.99 including free shipping. Apple charges $1999 for this model, so Amazon’s price is represents a $250 savings. Read more
Deal alert! 13-inch 2.0GHz MacBook Pros for $...
B&H Photo has the new 2016 13″ 2.0GHz non-Touch Bar MacBook Pros in stock today and on sale for $225 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro... Read more
Free LibreOffice Portable 5.2.4 Complete Offi...
PortableApps.com and The Document Foundation have announce the release of LibreOffice Portable 5.2.4. LibreOffice Portable is an Open Source full-featured office suite — including a word processor,... Read more
Apple Planning Three New Tablets For 2017 – D...
Digitimes’ Rebecca Kuo and Joseph Tsai say that unnamed insider sources report Apple having three new tablets in the pipeline for 2017 release: a 9.7-inch model with a friendly price range, a new mid... Read more
Roundup of 15-inch Touch Bar MacBook Pro sale...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... 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
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Manhattan, NY Introduction: We have immediate job openings for several Desktop Support Technicians with one of our most Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Stamford, CT We have immediate job openings for several Desktop Support Technicians with one of our most well-known 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
*Apple* Site Security Manager - Apple (Unite...
# Apple Site Security Manager Job Number: 54692472 Culver City, California, United States Posted: Jan. 19, 2017 Weekly Hours: 40.00 **Job Summary** The Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.