if (my $num = /\[/.../\]/)

##</code><code>##

   if (my $num = $_ =~ /\[/.../\]/)

##</code><code>##

my $regexp='\[/.../\]';
use YAPE::Regex::Explain;
my $exp = YAPE::Regex::Explain->new($regexp)->explain;
print $exp;

##</code><code>##

The regular expression:

(?-imsx:\[/.../\])

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):
----------------------------------------------------------------------
  \[                       '['
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  \]                       ']'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------