the regex match anything anytime ('.*') then a unix path separator ( '\/' i.e. '/' but escaped, because is the same separator used in this regex), then capture (and put it in $1) followed by anychar one time ( '.' ERROR: the dot must be escaped if you want a real lonely dot!) and endly a 'lib' string. All the matched string will be subsituted by what is in the captured variable ($1).

But YAPE::Regex::Explain is our friend!

perl -MYAPE::Regex::Explain -e "my $rex = qr(s/.*\/(.*).lib/$1/); prin
+t YAPE::Regex::Explain->new($rex)->explain"
The regular expression:

(?-imsx:s/.*\/(.*).lib//)

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):
----------------------------------------------------------------------
  s/                       's/'
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \/                       '/'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  lib//                    'lib//'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

L*
UPDATE: as stated by wise monks below the module does not handle substitutions and in the case above produce somehow misleading output (as lib//). So better to reduce the regex to a non substituiton one and inspect the output to see where the error is (at the dot). 
perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new('.*\/(
+.*).lib')->explain"
The regular expression:

(?-imsx:.*\/(.*).lib)

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):
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \/                       '/'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  lib                      'lib'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]
There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

In reply to Re: Regex explanation by Discipulus
in thread Regex explanation by ElectroRed

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