[core:text/regex] Fix #6323 and add test case

Thanks to @GPotoshin for the fix.

core/text/regex/virtual_machine/virtual_machine.odin

@@ -203,18 +203,22 @@ add_thread :: proc(vm: ^Machine, saved: ^[2 * common.MAX_CAPTURE_GROUPS]int, pc:
 			}
 		case .Assert_Word_Boundary:
 			sp := vm.string_pointer+vm.current_rune_size
-			if sp == 0 || sp == len(vm.memory) {
+
+			left_is_wc := false
+			if sp > 0 {
+				left_is_wc = is_word_class(vm.current_rune)
+			}
+
+			right_is_wc := false
+			if sp < len(vm.memory) {
+				right_is_wc = is_word_class(vm.next_rune)
+			}
+
+			if left_is_wc != right_is_wc {
 				pc += size_of(Opcode)
 				continue
-			} else {
-				last_rune_is_wc := is_word_class(vm.current_rune)
-				this_rune_is_wc := is_word_class(vm.next_rune)
-
-				if last_rune_is_wc && !this_rune_is_wc || !last_rune_is_wc && this_rune_is_wc {
-					pc += size_of(Opcode)
-					continue
-				}
 			}
+
 		case .Assert_Non_Word_Boundary:
 			sp := vm.string_pointer+vm.current_rune_size
 			if sp != 0 && sp != len(vm.memory) {

tests/core/text/regex/test_core_text_regex.odin

@@ -1270,6 +1270,16 @@ iterator_vectors := []Iterator_Test{
 			{pos = {{0, 2}}, groups = {"a\n"}},
 		},
 	},
+	// https://github.com/odin-lang/Odin/issues/6323
+	// Test `\b` in iterator
+	{
+		"can, can't, 'can't'", `\b[a-z0-9']+\b`, {},
+		{
+			{pos = {{0, 3}},   groups = {"can"}},
+			{pos = {{5, 10}},  groups = {"can't"}},
+			{pos = {{13, 18}}, groups = {"can't"}},
+		},
+	}
 }
 
 @test