Because, of all the things you can do with the -D switch, debugging REs is the most commonly wanted when not debugging perl itself, you can do: 
$ perl -we'use re "debug"; qr/[\s*\W*]/'
Compiling REx `[\s*\W*]'
size 13 Got 108 bytes for offset annotations.
first at 1
   1: ANYOF[\0-/:-@[-\^`{-\377+utf8::IsSpacePerl !utf8::IsWord](13)
  13: END(0)
stclass `ANYOF[\0-/:-@[-\^`{-\377+utf8::IsSpacePerl !utf8::IsWord]' mi
+nlen 1
Offsets: [13]
        1[8] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 9[
+0]
Freeing REx: `"[\\s*\\W*]"'

[download]
even on a non-debugging perl.

In reply to Re: Compiling and Installing Perl by ysth
in thread Compiling and Installing Perl by xenchu

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