Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Python and Tcl - public course schedule [here]
Private courses on your site - see [here]
Please ask about maintenance training for Perl, PHP, Lua, etc
 
Possessive Regular Expression Matching - Perl, Objective C and some other languages

Cost of a house"I'm looking to spend between £200,000 and £225,000 on a new home" you say to the salesman and - guess what - you're offered something much nearer £225,000 that £200,000.

With Regular Expression matching, you can ask the question "do we have a match", and that returns a Yes / No flag - so it doesn't matter how it matches. But with regular expression matching to a string, you can also ask how it matches (i.e. "please return to me the bits of the incoming string which match each part of the pattern") and in that case the how does matter.

Let's see a series of examples, and I've chosen to use Perl. Here's a string:

  $source = qq%Please "press" the enter key gently, don't "hit" enter!\n%;

Using a default (that's a greedy match, counts such as + will match as many characters as possible, so:
  $source =~ /(\w+)\s*(\w+)/;
  print "We have $1 and $2\n";

will match as many letters as possible to the first \w+ - thats "Please". It will match the space to the \s*, but then fail at the double quote as it wants at least one word character. So it then steps backwards, matching just "Pleas" to the first \w+, nothing (no spaces) to the \s*, and the final e of the word Please to the second \w+. The result is
  We have Pleas and e

Using a sparse count - that's +? with the extra "?", we match as few characters as possible in each count. So:
  $source =~ /(\w+?)\s*(\w+)/;
  print "We have $1 and $2\n";

will match as few letters as possible to the \w+? - that's just "P". It then finds no spaces which satisfies the \s* and it matches lease to the final \w+. The result is
  We have P and lease

There's a third type of count - a possessive count - too. It was added at release 5.10 of Perl, and it's available in other regular expression engines too such as that in Objective C. It's a greedy count too, but with the difference that it will not step backwards to look for a shorter match once a longer one has been found for the particular count. To request a possessive match, add an extra + after the default count, so:
  $source =~ /(\w++)\s*(\w+)/;
  print "We have $1 and $2\n";

This will match the "Please" to \w++, the space to \s*, and will then fail as it tries to match the \w+ to the double quote. It will not step back in the way the default did, so it will start matching the \w++ to the word press. Once again, it will fail to match at the next double quote, and will move on rather than stepping back. It will match the \w++ to the word the, the space to \s*, and then (successfully!) the second \w+ to the word enter, giving a result
  We have the and enter

You'll note that in this example, the possessive count results in a dramatically different match - though that won't always be the case. In fact, the documentation states that the main purpose of this new count is to allow the programmer to write regular expression matches that run faster as needless backtracking and matching attempts can be avoided.

Possessive regular expressions won't get more than a brief mention on most of our courses, although we'll talk about them (and perhaps show a demonstration) on Perl for Larger Projects if delegates have run time concerns. We will also cover them on our Regular Expression Course.

The examples that I've used above are shown as a complete program on our web site - [here].

(written 2012-03-12)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C501 - C and C based languages - Objective C, XCode and iOS Resources
  [3649] A single action for multiple iPad / iPhone buttons, and animation - (2012-03-11)
  [3648] iPad and iPhone programming - our seminar weekend with Xcode - (2012-03-11)
  [3599] Seeing how Melksham has changed over the years, via an iPad - (2012-01-30)
  [3596] Want to learn iPad and iPhone programming? Come along and learn with me for free. - (2012-01-28)
  [3594] Back to Uni - (2012-01-26)

P212 - Perl - More on Character Strings
  [4452] Binary data handling - Python and Perl - (2015-03-09)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [608] Don't expose your regular expressions - (2006-02-15)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [583] Remember to process blank lines - (2006-01-31)
  [453] Commenting Perl regular expressions - (2005-09-30)

Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [2909] Be gentle rather than macho ... regular expression techniques - (2010-08-08)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1372] A taster PHP expression ... - (2007-09-30)


Back to
A single action for multiple iPad / iPhone buttons, and animation
Previous and next
or
Horse's mouth home
Forward to
Makefile - some basics, and a demonstration
Some other Articles
On a sunny afternoon in London
What is happening in 59 days time in Melksham?
A Complete makefile example
Makefile - some basics, and a demonstration
Possessive Regular Expression Matching - Perl, Objective C and some other languages
Along the brook - East Melksham to Melksham
Exploring Melksham with a film maker
Keeping our hotel looking like new, by using our gained experience
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2019: 404 The Spa • Melksham, Wiltshire • United Kingdom • SN12 6QL
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/3650_Pos ... uages.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat