$ perl -Mre=debug -E'$x=qr/[X0-9a-f]/i; $hr=qr/$x+ - $x+/x;'
Compiling REx "[X0-9a-f]"
Final program:
   1: ANYOF[0-9A-FXa-fx] (11)
  11: END (0)
stclass ANYOF[0-9A-FXa-fx] minlen 1             
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Final program:                                  
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)
  12: EXACT <-> (14)                           
  14: PLUS (25)                                
  15:   ANYOF[0-9A-FXa-fx] (0)
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANYOF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Freeing REx: "[X0-9a-f]"
$  

##</code><code>##

$ perl -Mre=debug -E'$hr=qr/(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+/x;'                         
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"                                           
Final program:
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)                  
  12: EXACT <-> (14)                            
  14: PLUS (25)
  15:   ANYOF[0-9A-FXa-fx] (0)                 
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANYOF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
$