diff --git a/include/pocketpy/common/str.h b/include/pocketpy/common/str.h
index d39c43af..cc3185ed 100644
--- a/include/pocketpy/common/str.h
+++ b/include/pocketpy/common/str.h
@@ -31,8 +31,8 @@ void pkpy_Str__dtor(pkpy_Str* self);
 pkpy_Str pkpy_Str__copy(const pkpy_Str* self);
 pkpy_Str pkpy_Str__concat(const pkpy_Str* self, const pkpy_Str* other);
 pkpy_Str pkpy_Str__concat2(const pkpy_Str* self, const char* other, int size);
-pkpy_Str pkpy_Str__substr(const pkpy_Str* self, int start);
-pkpy_Str pkpy_Str__substr2(const pkpy_Str* self, int start, int size);
+pkpy_Str pkpy_Str__slice(const pkpy_Str* self, int start);
+pkpy_Str pkpy_Str__slice2(const pkpy_Str* self, int start, int stop);
 pkpy_Str pkpy_Str__lower(const pkpy_Str* self);
 pkpy_Str pkpy_Str__upper(const pkpy_Str* self);
 pkpy_Str pkpy_Str__escape(const pkpy_Str* self, char quote);
diff --git a/include/pocketpy/common/str.hpp b/include/pocketpy/common/str.hpp
index f43fb16a..1b2ea5a6 100644
--- a/include/pocketpy/common/str.hpp
+++ b/include/pocketpy/common/str.hpp
@@ -149,12 +149,16 @@ struct Str: pkpy_Str {
         return std::string(pkpy_Str__data(this), size);
     }
 
-    Str substr(int start, int size) const{
-        return pkpy_Str__substr2(this, start, size);
+    Str slice(int start, int stop) const{
+        return pkpy_Str__slice2(this, start, stop);
+    }
+
+    Str slice(int start) const{
+        return pkpy_Str__slice(this, start);
     }
 
     Str substr(int start) const{
-        return pkpy_Str__substr(this, start);
+        return pkpy_Str__slice(this, start);
     }
 
     Str strip(bool left, bool right, const Str& chars) const;
diff --git a/src/common/str.c b/src/common/str.c
index 905a9f83..df33c737 100644
--- a/src/common/str.c
+++ b/src/common/str.c
@@ -90,13 +90,14 @@ pkpy_Str pkpy_Str__concat2(const pkpy_Str *self, const char *other, int size){
     return retval;
 }
 
-pkpy_Str pkpy_Str__substr(const pkpy_Str *self, int start){
-    return pkpy_Str__substr2(self, start, self->size - start);
+pkpy_Str pkpy_Str__slice(const pkpy_Str *self, int start){
+    return pkpy_Str__slice2(self, start, self->size);
 }
 
-pkpy_Str pkpy_Str__substr2(const pkpy_Str *self, int start, int size){
+pkpy_Str pkpy_Str__slice2(const pkpy_Str *self, int start, int stop){
     pkpy_Str retval;
-    pkpy_Str__ctor2(&retval, pkpy_Str__data(self) + start, size);
+    if(stop < start) stop = start;
+    pkpy_Str__ctor2(&retval, pkpy_Str__data(self) + start, stop - start);
     return retval;
 }
 
@@ -174,6 +175,45 @@ pkpy_Str pkpy_Str__escape(const pkpy_Str* self, char quote){
     return retval;
 }
 
+// pkpy_Str pkpy_Str__strip(const pkpy_Str *self, bool left, bool right){
+//     const char* data = pkpy_Str__data(self);
+//     if(self->is_ascii) {
+//         int L = 0;
+//         int R = self->size;
+//         if(left) {
+//             while(L < R && (data[L] == ' ' || data[L] == '\t' || data[L] == '\n' || data[L] == '\r'))
+//                 L++;
+//         }
+//         if(right) {
+//             while(L < R && (data[R - 1] == ' ' || data[R - 1] == '\t' || data[R - 1] == '\n' || data[R - 1] == '\r'))
+//                 R--;
+//         }
+//         return pkpy_Str__substr2(self, L, R - L);
+//     } else {
+//         pkpy_Str tmp;
+//         pkpy_Str__ctor(&tmp, " \t\n\r");
+//         pkpy_Str retval = pkpy_Str__strip2(self, left, right, &tmp);
+//         pkpy_Str__dtor(&tmp);
+//         return retval;
+//     }
+// }
+
+// pkpy_Str pkpy_Str__strip2(const pkpy_Str *self, bool left, bool right, const pkpy_Str *chars){
+//     int L = 0;
+//     int R = pkpy_Str__u8_length(self);
+//     pkpy_Str tmp;
+//     if(left) {
+//         tmp = pkpy_Str__u8_getitem(self, L);
+//         while(L < R && chars.index(u8_getitem(L)) != -1)
+//             L++;
+//     }
+//     if(right) {
+//         while(L < R && chars.index(u8_getitem(R - 1)) != -1)
+//             R--;
+//     }
+//     return pkpy_Str__u8_slice(self, L, R, 1);
+// }
+
 pkpy_Str pkpy_Str__replace(const pkpy_Str *self, char old, char new_){
     pkpy_Str retval = pkpy_Str__copy(self);
     char* p = (char*)pkpy_Str__data(&retval);
@@ -190,13 +230,13 @@ pkpy_Str pkpy_Str__replace2(const pkpy_Str *self, const pkpy_Str *old, const pkp
     while(true) {
         int i = pkpy_Str__index(self, old, start);
         if(i == -1) break;
-        pkpy_Str tmp = pkpy_Str__substr2(self, start, i - start);
+        pkpy_Str tmp = pkpy_Str__slice2(self, start, i);
         c11_vector__extend(char, &buffer, pkpy_Str__data(&tmp), tmp.size);
         pkpy_Str__dtor(&tmp);
         c11_vector__extend(char, &buffer, pkpy_Str__data(new_), new_->size);
         start = i + old->size;
     }
-    pkpy_Str tmp = pkpy_Str__substr2(self, start, self->size - start);
+    pkpy_Str tmp = pkpy_Str__slice2(self, start, self->size);
     c11_vector__extend(char, &buffer, pkpy_Str__data(&tmp), tmp.size);
     pkpy_Str__dtor(&tmp);
     c11_vector__push(char, &buffer, '\0');
@@ -221,10 +261,8 @@ int pkpy_Str__cmp2(const pkpy_Str *self, const char *other, int size){
 
 pkpy_Str pkpy_Str__u8_getitem(const pkpy_Str *self, int i){
     i = pkpy_Str__unicode_index_to_byte(self, i);
-    return pkpy_Str__substr2(
-        self, i,
-        pkpy_utils__u8_header(pkpy_Str__data(self)[i], false)
-    );
+    int size = pkpy_utils__u8_header(pkpy_Str__data(self)[i], false);
+    return pkpy_Str__slice2(self, i, i + size);
 }
 
 pkpy_Str pkpy_Str__u8_slice(const pkpy_Str *self, int start, int stop, int step){
diff --git a/src/common/str.cpp b/src/common/str.cpp
index 6471d7b2..d30dcb2f 100644
--- a/src/common/str.cpp
+++ b/src/common/str.cpp
@@ -50,7 +50,7 @@ Str Str::strip(bool left, bool right) const {
             while(L < R && (data[R - 1] == ' ' || data[R - 1] == '\t' || data[R - 1] == '\n' || data[R - 1] == '\r'))
                 R--;
         }
-        return substr(L, R - L);
+        return slice(L, R);
     } else {
         return strip(left, right, " \t\n\r");
     }
diff --git a/src/compiler/expr.cpp b/src/compiler/expr.cpp
index ed0d38ce..9327ea1a 100644
--- a/src/compiler/expr.cpp
+++ b/src/compiler/expr.cpp
@@ -414,11 +414,11 @@ void FStringExpr::_load_simple_expr(CodeEmitContext* ctx, Str expr) {
         switch(expr.end()[-1]) {
             case 'r':
                 repr = true;
-                expr = expr.substr(0, expr.size - 2);
+                expr = expr.slice(0, expr.size - 2);
                 break;
             case 's':
                 repr = false;
-                expr = expr.substr(0, expr.size - 2);
+                expr = expr.slice(0, expr.size - 2);
                 break;
             default: break;  // nothing happens
         }
@@ -472,7 +472,7 @@ void FStringExpr::emit_(CodeEmitContext* ctx) {
         if(flag) {
             if(src[j] == '}') {
                 // add expression
-                Str expr = src.substr(i, j - i);
+                Str expr = src.slice(i, j);
                 // BUG: ':' is not a format specifier in f"{stack[2:]}"
                 int conon = expr.index(":");
                 if(conon >= 0) {
@@ -485,7 +485,7 @@ void FStringExpr::emit_(CodeEmitContext* ctx) {
                             break;
                         }
                     if(ok) {
-                        _load_simple_expr(ctx, expr.substr(0, conon));
+                        _load_simple_expr(ctx, expr.slice(0, conon));
                         ctx->emit_(OP_FORMAT_STRING, ctx->add_const_string(spec.sv()), line);
                     } else {
                         // ':' is not a spec indicator
@@ -527,7 +527,7 @@ void FStringExpr::emit_(CodeEmitContext* ctx) {
                 i = j;
                 while(j < src.size && src[j] != '{' && src[j] != '}')
                     j++;
-                Str literal = src.substr(i, j - i);
+                Str literal = src.slice(i, j);
                 ctx->emit_(OP_LOAD_CONST, ctx->add_const_string(literal.sv()), line);
                 count++;
                 continue;  // skip j++
@@ -538,7 +538,7 @@ void FStringExpr::emit_(CodeEmitContext* ctx) {
 
     if(flag) {
         // literal
-        Str literal = src.substr(i, src.size - i);
+        Str literal = src.slice(i, src.size);
         ctx->emit_(OP_LOAD_CONST, ctx->add_const_string(literal.sv()), line);
         count++;
     }
diff --git a/src/interpreter/iter.cpp b/src/interpreter/iter.cpp
index 089df39c..18c484d0 100644
--- a/src/interpreter/iter.cpp
+++ b/src/interpreter/iter.cpp
@@ -51,7 +51,7 @@ void StringIter::_register(VM* vm, PyObject* mod, PyObject* type) {
         int start = self.i;
         int len = pkpy_utils__u8_header(s[self.i], false);
         self.i += len;
-        vm->s_data.push(VAR(s.substr(start, len)));
+        vm->s_data.push(VAR(s.slice(start, self.i)));
         return 1;
     });
 }
diff --git a/src/interpreter/vm.cpp b/src/interpreter/vm.cpp
index 392af7e5..ce4d1114 100644
--- a/src/interpreter/vm.cpp
+++ b/src/interpreter/vm.cpp
@@ -623,7 +623,7 @@ PyVar VM::__format_object(PyVar obj, Str spec) {
         case 'd':
         case 's':
             type = spec.end()[-1];
-            spec = spec.substr(0, spec.length() - 1);
+            spec = spec.slice(0, spec.length() - 1);
             break;
         default: type = ' '; break;
     }
@@ -660,9 +660,9 @@ PyVar VM::__format_object(PyVar obj, Str spec) {
             if(dot == 0) {
                 width = -1;
             } else {
-                width = std::stoi(spec.substr(0, dot).str());
+                width = std::stoi(spec.slice(0, dot).str());
             }
-            precision = std::stoi(spec.substr(dot + 1).str());
+            precision = std::stoi(spec.slice(dot + 1).str());
         } else {
             width = std::stoi(spec.str());
             precision = -1;
@@ -761,7 +761,7 @@ static std::string _opcode_argstr(VM* vm, int i, Bytecode byte, const CodeObject
 
 Str VM::disassemble(CodeObject_ co) {
     auto pad = [](const Str& s, const int n) {
-        if(s.length() >= n) return s.substr(0, n);
+        if(s.length() >= n) return s.slice(0, n);
         return s + std::string(n - s.length(), ' ');
     };