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].

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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)

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

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)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)

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)

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Catering in Syracuse, the Saigon Cafe, stolen images and Christmas
Thank you - and Happy Christmas
AND and OR operators - what is the difference between logical and bitwise varieties?
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
Contrast in pictures
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