Commenting a Perl Regular Expression

The x modifier on the end of a Perl regular expression causes all spaces in the regular expression to be treated as comments (rather than matching exactly). This means that you can lay out your regular expressions much more cleanly.

And wherever you're allowed white space, you can add comments from # to end of line. So the following code (for finding Belgian car registrations in a line of input) is valid:

while ($info = <>) {

        while ($info =~ /

                \b

                ([A-Z]{3})      # capture letters to $1

                -?

                (\d{3})         # capture digits to $2

                \b

                /gx             ) {

           print "Yes, $2, $1\n";

        }

        print "And that's your lot\n";

}

Note that you should NOT use the regular expression terminator (/ in our example) within the comments you add ... and if you REALLY want to match a space, you should now use \s for any space character or \x20 for a single space.
(written 2007-06-12, updated 2007-06-13)

Associated topics are indexed under

P212 - Perl - More on Character Strings
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  [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)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [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)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [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)

Q806 - Regular Expression Cookbook
  [3788] Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [2804] Regular Expression Myths - (2010-06-13)
  [2727] Making a Lua program run more than 10 times faster - (2010-04-16)
  [2702] First and last match with Regular Expressions - (2010-04-02)
  [2608] Search and replace in Ruby - Ruby Regular Expressions - (2010-01-31)
  [2563] Efficient debugging of regular expressions - (2010-01-04)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [1840] Validating Credit Card Numbers - (2008-10-14)
  [672] Keeping your regular expressions simple - (2006-04-05)

556c

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