D:\tmp>perl -Mre=debug -e"'foo' =~ m{ ( o? )* }x;"
Compiling REx " ( o? )* "
Final program:
   1: CURLYX[0]{0,INFTY} (12)
   3:   OPEN1 (5)
   5:     CURLY{0,1} (9)
   7:       EXACT <o> (0)
   9:   CLOSE1 (11)
  11: WHILEM[1/1] (0)
  12: NOTHING (13)
  13: END (0)
minlen 0
Matching REx " ( o? )* " against "foo"
   0 <> <foo>                |   0| 1:CURLYX[0]{0,INFTY}(12)
   0 <> <foo>                |   1|  11:WHILEM[1/1](0)
                             |   1|  WHILEM: matched 0 out of 0..65535
   0 <> <foo>                |   2|   3:OPEN1(5)
   0 <> <foo>                |   2|   5:CURLY{0,1}(9)
                             |   2|   EXACT <o> can match 0 times out of 1...
   0 <> <foo>                |   3|    9:CLOSE1(11)
   0 <> <foo>                |   3|    11:WHILEM[1/1](0)
                             |   3|    WHILEM: matched 1 out of 0..65535
                             |   3|    WHILEM: empty match detected, trying continuation... <----    HERE                                                                    
   0 <> <foo>                |   4|     12:NOTHING(13)
   0 <> <foo>                |   4|     13:END(0)
Match successful!
Freeing REx: " ( o? )* "

D:\tmp>