Use EPTR to assert pointer is after string beginning

Author: khwilliamson

embed.fnc

@@ -185,6 +185,9 @@
 :   final byte that EPTR points to.  There are functions that do that, and they
 :   use EPTRQ to declare that the final byte is fair game.
 :
+:   Finally, this same ability can be used to verify that a pointer to the
+:   middle of the string is beyond the string beginning.
+:
 :   For a numeric argument, you may specify that it can't be 0 by using 'NZ'
 :
 :   regen/embed.pl may automatically add further checking for any argument as
@@ -5748,19 +5751,19 @@ ERS	|WB_enum|advance_one_WB_|NN U8 **curpos 			\
 				|const bool utf8_target 		\
 				|const bool skip_Extend_Format
 ERS	|GCB_enum|backup_one_GCB|NN const U8 * const strbeg		\
-				|NN U8 **curpos 			\
+				|EPTRQ U8 **curpos			\
 				|const bool utf8_target
 ERS	|LB_enum|backup_one_LB_ |NN const U8 * const strbeg		\
-				|NN U8 **curpos 			\
+				|EPTRQ U8 **curpos			\
 				|const bool utf8_target 		\
 				|bool skip_CM_ZWJ
 ERS	|SB_enum|backup_one_SB	|NN const U8 * const strbeg		\
-				|NN U8 **curpos 			\
+				|EPTRQ U8 **curpos			\
 				|const bool utf8_target
 ERS	|WB_enum|backup_one_WB_but_over_Extend_FO			\
 				|NN WB_enum *previous			\
 				|NN const U8 * const strbeg		\
-				|NN U8 **curpos 			\
+				|EPTRQ U8 **curpos			\
 				|const bool utf8_target
 EWi	|void	|capture_clear	|NN regexp *rex 			\
 				|U16 from_ix				\