REG_ESUBREG      2256 lib/regcomp.c  	  *err = REG_ESUBREG;
REG_ESUBREG       833 src/regex-emacs.c    [REG_ESUBREG] = "Invalid back reference",
REG_ESUBREG      2581 src/regex-emacs.c 		  FREE_STACK_RETURN (REG_ESUBREG);