Another really nice feature is use re 'debug';:

#!/usr/bin/perl
use warnings;
use strict;

use re 'debug';

"this" =~ /(?<!x)i?/;
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Output:

Compiling REx "(?<!x)i?"
Final program:
   1: UNLESSM[-1] (7)
   3:   EXACT <x> (5)
   5:   SUCCEED (0)
   6: TAIL (7)
   7: CURLY {0,1} (11)
   9:   EXACT <i> (0)
  11: END (0)
minlen 0 
Matching REx "(?<!x)i?" against "this"
   0 <> <this>               |  1:UNLESSM[-1](7)
   0 <> <this>               |  7:CURLY {0,1}(11)
                                  EXACT <i> can match 0 times out of 1
+...
   0 <> <this>               | 11:  END(0)
Match successful!
Freeing REx: "(?<!x)i?"
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