$ perl -Mre=debug -e 'qr/(0)+|(1)+/' Compiling REx "(0)+|(1)+" Final program: 1: BRANCH (12) 2: CURLYN[1] {1,32767} (23) 4: NOTHING (6) 6: EXACT <0> (0) 10: WHILEM (0) 11: NOTHING (23) 12: BRANCH (FAIL) 13: CURLYN[2] {1,32767} (23) 15: NOTHING (17) 17: EXACT <1> (0) 21: WHILEM (0) 22: NOTHING (23) 23: END (0) minlen 1 Freeing REx: "(0)+|(1)+"