"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)

 Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1372] A taster PHP expression ... - (2007-09-30)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [2909] Be gentle rather than macho ... regular expression techniques - (2010-08-08)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)

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

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

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

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