There are some things that it's just plain difficult to find good, practical examples to show delegates on training course. "Solutions looking for problems" I call them sometimes. And one such group are lookahead and lookbehind in regular expression.

However, I think I did a bit better yesterday ...

Take the lines
  2598 LOT BL6 4JP SD 673086 53.5729974812 -2.4937754345 Lostock Parkway 118752 125475 126934 140033 208136 209168
and
  1090 AVP NULL NULL NULL NULL Aylesbury Vale Parkway NULL NULL NULL NULL 13042 49212
and a requirement to match "Parkway" and the following number, BUT ONLY if the line doesn't contain the word NULL later on.

Here's the demonstration solution - using negative lookahead:
  if (preg_match('/\t([^\t]+Parkway)(?!.*NULL)\t(\d+)/',$line,$gotten)) {

Sadly, I still advocate that it's better to perform this matching in two simpler regular expressions rather than one more complex one. Two easier statements will be quicker to code and test, easier to understand and update later, and will probably run faster too:
  if (preg_match('/Parkway.*NULL/',$line)) continue;
  if (preg_match('/\t([^\t]+Parkway)\t(\d+)/',$line,$gotten)) {

Full source code - [here].
(written 2012-06-30)

 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)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)

H107 - String Handling in PHP
  [31] Here documents - (2004-08-28)
  [54] PHP and natural sorting - (2004-09-19)
  [337] the array returned by preg_match_all - (2005-06-06)
  [422] PHP Magic Quotes - (2005-08-22)
  [463] Splitting the difference - (2005-10-13)
  [493] Running a Perl script within a PHP page - (2005-11-12)
  [558] Converting between acres and hectares - (2006-01-08)
  [560] The fencepost problem - (2006-01-10)
  [574] PHP - dividing a string up into pieces - (2006-01-23)
  [589] Robust PHP user inputs - (2006-02-03)
  [608] Don't expose your regular expressions - (2006-02-15)
  [642] How similar are two words - (2006-03-11)
  [716] Evaluating arithmetic expressions in configuration files - (2006-05-10)
  [1008] Date conversion - PHP - (2006-12-26)
  [1058] PHP Regular expression to extrtact link and text - (2007-01-31)
  [1195] Regular Express Primer - (2007-05-20)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1372] A taster PHP expression ... - (2007-09-30)
  [1533] Short and sweet and sticky - PHP form input - (2008-02-06)
  [1603] Do not SHOUT and do not whisper - (2008-04-06)
  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1799] Regular Expressions in PHP - (2008-09-16)
  [2046] Finding variations on a surname - (2009-02-17)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [2238] Handling nasty characters - Perl, PHP, Python, Tcl, Lua - (2009-06-14)
  [2629] Curly braces within double quoted strings in PHP - (2010-02-09)
  [3020] Handling (expanding) tabs in PHP - (2010-10-29)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [3515] PHP - moving from ereg to preg for regular expressions - (2011-11-11)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [3534] Learning to program in PHP - Regular Expression and Associative Array examples - (2011-12-01)
  [3788] Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30)
  [3789] More than just matching with a regular expression in PHP - (2012-06-30)
  [4071] Setting up strings in PHP - (2013-04-27)
  [4072] Splitting the difference with PHP - (2013-04-27)

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