Solution looking for a problem? Lookahead and Lookbehind

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)

Associated topics are indexed under

Q805 - Object Orientation and General technical topics - Advanced Regular Expression Components
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [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)

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

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