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Looking ahead and behind in a Regular Expression

Regular expressions in Perl and PHP include facilities called zero width assertions, zero width lookahead and lookbehinds. A case of jargon that looks almost calculated to confuse?

Zero width assertions are where a regular expression matches some sort of condition in the line, without actually consuming any characters from the incoming string - the three most common examples are ^ (must be at start of string), $ (must be at end of string) and \b (must be at word boundary).

There are times when you may wish to say "if followed by", "if not followed by", "if following" and "if NOT following" in a regular expression match, but to not actually move backward or forward over the incoming string - for example, in a spell checker I was writing yesterday (source, read about it and try it out) I was looking to split my incoming string at each word boundary, but only if NOT following or followed by a single quote. And, crucially, the single quote character was not to be included in the matched string itself - I was just saying "no break here" in the case of words like hasn't and I'll. This is a requirement for a zero width negative look behind written (?<!') and a zero width negative look ahead written (?!').

Here's the complete regular expression of my example:
$elements = preg_split("/\b(?<!')(?!')/",$page);

Footnote - Zero width positive lookaheads are written (?=xx) and zero width positive look behinds are written (?<=xx), where xx is the expression that you're looking back or forward to match
(written 2006-05-22)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
H107 - String Handling in PHP
  [4072] Splitting the difference with PHP - (2013-04-27)
  [4071] Setting up strings in PHP - (2013-04-27)
  [3790] Solution looking for a problem? Lookahead and Lookbehind - (2012-06-30)
  [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)

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)


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