Allow me to introduce you to a handy CPAN module that I discovered by way of another kind monk some time ago (paying it forward if you will, even though I no longer recall who pointed me in this direction)

YAPE::Regex::Explain -- an handy tool for explaining regular expressions. I use it frequently when I come across a regex that I don't understand. Here's a quick example based on your request:

#!/usr/bin/perl -wl

use YAPE::Regex::Explain;

my $regex =  qr/[a-zA-Z][a-zA-Z][a-zA-Z]\d\d,\s+\interlock/;

print YAPE::Regex::Explain->new($regex)->explain;
[download]

You will see the \interlock yeilds an "Unrecognized escape '\i' passed through" --- which YAPE ignores.

moritz pointed this out above.

YAPE will produce the following output. (Notice that the '\i' has been removed from the regex.

The regular expression:

(?-imsx:[a-zA-Z][a-zA-Z][a-zA-Z]\d\d,\s+interlock)

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-Z]                 any character of: 'a' to 'z', 'A' to 'Z'
----------------------------------------------------------------------
  [a-zA-Z]                 any character of: 'a' to 'z', 'A' to 'Z'
----------------------------------------------------------------------
  [a-zA-Z]                 any character of: 'a' to 'z', 'A' to 'Z'
----------------------------------------------------------------------
  \d                       digits (0-9)
----------------------------------------------------------------------
  \d                       digits (0-9)
----------------------------------------------------------------------
  ,                        ','
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  interlock                'interlock'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

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