Tip #9 from the Basic debugging checklist:

Demystify regular expressions by installing and using the CPAN module YAPE::Regex::Explain

So:

#! perl
use strict;
use warnings;
use YAPE::Regex::Explain;

my $re = qr/(?<=module )$ARGV[2].*?([\\(;])/;

print YAPE::Regex::Explain->new($re)->explain();
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Output:

15:40 >perl 566_SoPW.pl FILE OLD NEW
The regular expression:

(?-imsx:(?<=module )NEW.*?([\\(;]))

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):
----------------------------------------------------------------------
  (?<=                     look behind to see if there is:
----------------------------------------------------------------------
    module                   'module '
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  NEW                      'NEW'
----------------------------------------------------------------------
  .*?                      any character except \n (0 or more times
                           (matching the least amount possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [\\(;]                   any character of: '\\', '(', ';'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

15:40 >
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See also perlretut and perlre.

Hope that helps,

It solves my issue but fails in the scenario when i have module this is in between OLD(A, B, Y); , i even want my script to change it to, module this is in between NEW(A, B, Y); hence i tried doing this (?<=module .*)$ARGV2.*?(\\(;)"; which doesnt work and shell says Variable length lookbehind not implemented in regex; marked by <-- HERE in m/(?<=module .*)NEW.*?(\(;) <-- HERE / at script.pl line 23.. How can i implement this?

ohk... i didnt knew about this feature that explains like this. Another important point i wish to make, I want to ignore // lines. Suppose somebody writes something like // module OLD(Y, A, B); would my $string not match and make script behavior wrong?? thats why i was trying to give ^module (WHICH MAKE SURE THAT ITS NOT A COMMENT). WHITESPACE AND TABS ARE OK BUT NO // BEFORE matching expression

next if m{\A//}
J -