Perl: Explanation of your regular expression

Perl CPAN has a module called "YAPE::Regex::Explain" which will help you to understand what is a perl regular expression doing.

Lets say you have a Regex [Za-z0-9]+ Here is the sample code if you want to get the explanation of your regex.

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new('[A-Za-z0-9]+')->explain;


This simple code will output as below on your screen, and which is easily understandable to get what is going on.

The regular expression:

(?-imsx:[A-Za-z0-9]+)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  [A-Za-z0-9]+             any character of: 'A' to 'Z', 'a' to 'z',
                           '0' to '9' (1 or more times (matching the
                           most amount possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

Lets try with a bit complex regular expression ST[^;]*(?:[A-Za-z0-9]+|[\.,\s]+)ET

The code for explaining for REGEX will be,

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new('ST[^;]*(?:[A-Za-z0-9]+|[\.,\s]+)ET')->explain;

Output is as below, and which is damn understandable.

The regular expression:

(?-imsx:ST[^;]*(?:[A-Za-z0-9]+|[\.,\s]+)ET)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ST                       'ST'
----------------------------------------------------------------------
  [^;]*                    any character except: ';' (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    [A-Za-z0-9]+             any character of: 'A' to 'Z', 'a' to
                             'z', '0' to '9' (1 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    [\.,\s]+                 any character of: '\.', ',', whitespace
                             (\n, \r, \t, \f, and " ") (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  ET                       'ET'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

Running the code from Command Line:

If you don't like to write a code for this, you can do it from your command line. Just one liner perl script. For above expression the code will be as below.

perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new('ST[^;]*(?:[A-Za-z0-9]+|[\.,\s]+)ET')->explain;"

How to install the module:

To install the module from your command prompt,
ppm install YAPE::Regex::Explain

And then type
ppm install YAPE::Regex

This installation can be done using cpan too.