$ perl -Mre=debug -le " q/a a/ =~ /(.\b)\1/ "
Compiling REx "(.\b)\1"
Final program:
   1: OPEN1 (3)
   3:   REG_ANY (4)
   4:   BOUND (5)
   5: CLOSE1 (7)
   7: REF1 (9)
   9: END (0)
minlen 1
Matching REx "(.\b)\1" against "a a"
   0 <> <a a>                |  1:OPEN1(3)
   0 <> <a a>                |  3:REG_ANY(4)
   1 <a> < a>                |  4:BOUND(5)
   1 <a> < a>                |  5:CLOSE1(7)
   1 <a> < a>                |  7:REF1(9)
                                  failed...
   1 <a> < a>                |  1:OPEN1(3)
   1 <a> < a>                |  3:REG_ANY(4)
   2 <a > <a>                |  4:BOUND(5)
   2 <a > <a>                |  5:CLOSE1(7)
   2 <a > <a>                |  7:REF1(9)
                                  failed...
   2 <a > <a>                |  1:OPEN1(3)
   2 <a > <a>                |  3:REG_ANY(4)
   3 <a a> <>                |  4:BOUND(5)
   3 <a a> <>                |  5:CLOSE1(7)
   3 <a a> <>                |  7:REF1(9)
                                  failed...
   3 <a a> <>                |  1:OPEN1(3)
   3 <a a> <>                |  3:REG_ANY(4)
                                  failed...
Match failed
Freeing REx: "(.\b)\1"