Looking ahead and behind in Regular Expressions

Look-ahead and look-behind are a way of "double matching" in a regular expression. If you're at a certain point in the match and you think "the next bit should conform to xxx and at the same time it should conform to yyy" then you can describe xxx via a look-ahead, and follow that with matching yyy in the usual way.

Using Perl syntax for this example:
$journey =~ /\w+(?=\w+,)ing.on/;
This says ... ""I'm looking for word characters. They are to be followed by more word characters ending in a comma. They are also to be followed (a second match against the same part in the incoming string) by "ing?on" where "?" is any one character.""

You can look ahead and look behind. And you can negate the result too. That's actually much more useful that the positive look ahead - allowing you to exclude special cases:
$journey =~ /\w+(?!ingt)ing.on/;
Which says ... ""NOT ingt but ing[somethingelse]on"".

There's example of all four possibilities (lookahead, lookbehind, positive and negative in each case) in a new example - written as a follow up to a question on yesterday's Perl for Larger Projects course - source code [here]. It's been quite the season for lookahead / lookbehind - there are other new examples in Python [here] and [here].

Are these facilities useful? Yes - on some occasions they can be, but there are often better alternatives. If you look at my simple examples, the same thing could be achieved in a much more straightforward way using more commonly understood regular expression elements, which will be easier for people less into the depths of regular expressions to support. And it's often very much the case that two simple regular expression matches are better (faster, easier to maintain) that one complicated and obtuse one.
(written 2010-12-23, updated 2010-12-24)

Associated topics are indexed under

Q805 - Object Orientation and General technical topics - Advanced Regular Expression Components
  [3790] Solution looking for a problem? Lookahead and Lookbehind - (2012-06-30)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [2909] Be gentle rather than macho ... regular expression techniques - (2010-08-08)
  [728] Looking ahead and behind in a Regular Expression - (2006-05-22)

Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [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)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)

P212 - Perl - More on Character Strings
  [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)
  [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)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [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)
  [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)

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Catering in Syracuse, the Saigon Cafe, stolen images and Christmas
Thank you - and Happy Christmas
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The week before Christmas
Looking ahead and behind in Regular Expressions - double matching
Perl - database access - DBD, DBI and DBIx modules
Learning Object Orientation in Perl through bananas and perhaps Moose
Making Perl class definitions more conventional and shorter
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