diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3c37b45f..7fd9bb72 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Breaking changes
 ----------------
 
 * `Engine::disable_doc_comments` is removed because doc-comments are now placed under the `metadata` feature flag.
+* Registering a custom syntax now only requires specifying whether the `Scope` is adjusted (i.e. whether variables are added or removed). There is no need to specify the number of variables added/removed.
 
 New features
 ------------
@@ -16,6 +17,11 @@ New features
 * A new internal feature `no_smartstring` to turn off `SmartString` for those rare cases that it is needed.
 * `DynamicReadLock` and `DynamicWriteLoc` are exposed under `internals`.
 
+Enhancements
+------------
+
+* Registering a custom syntax now only requires specifying whether the `Scope` is adjusted (i.e. whether variables are added or removed). This allows more flexibility for cases where the number of new variables declared depends on internal logic.
+
 
 Version 0.20.1
 ==============
diff --git a/src/ast.rs b/src/ast.rs
index c0f096c6..060875ca 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1357,8 +1357,8 @@ impl Stmt {
 pub struct CustomExpr {
     /// List of keywords.
     pub keywords: StaticVec<Expr>,
-    /// Delta number of variables in the scope.
-    pub scope_delta: isize,
+    /// Is the current [`Scope`][crate::Scope] modified?
+    pub scope_changed: bool,
     /// List of tokens actually parsed.
     pub tokens: StaticVec<Identifier>,
 }
diff --git a/src/optimize.rs b/src/optimize.rs
index 93878e85..25928c1a 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -1024,7 +1024,7 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
 
         // Custom syntax
         Expr::Custom(x, _) => {
-            if x.scope_delta != 0 {
+            if x.scope_changed {
                 state.propagate_constants = false;
             }
             x.keywords.iter_mut().for_each(|expr| optimize_expr(expr, state));
diff --git a/src/parser.rs b/src/parser.rs
index aaa9a11e..5cb722bc 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1848,22 +1848,12 @@ fn parse_custom_syntax(
     let mut tokens: StaticVec<_> = Default::default();
 
     // Adjust the variables stack
-    match syntax.scope_delta {
-        delta if delta > 0 => {
-            // Add enough empty variable names to the stack.
-            // Empty variable names act as a barrier so earlier variables will not be matched.
-            // Variable searches stop at the first empty variable name.
-            let empty = state.get_identifier("");
-            state.stack.resize(
-                state.stack.len() + delta as usize,
-                (empty, AccessMode::ReadWrite),
-            );
-        }
-        delta if delta < 0 && state.stack.len() <= delta.abs() as usize => state.stack.clear(),
-        delta if delta < 0 => state
-            .stack
-            .truncate(state.stack.len() - delta.abs() as usize),
-        _ => (),
+    if syntax.scope_changed {
+        // Add an empty variable name to the stack.
+        // Empty variable names act as a barrier so earlier variables will not be matched.
+        // Variable searches stop at the first empty variable name.
+        let empty = state.get_identifier("");
+        state.stack.push((empty, AccessMode::ReadWrite));
     }
 
     let parse_func = &syntax.parse;
@@ -1936,7 +1926,7 @@ fn parse_custom_syntax(
         Box::new(CustomExpr {
             keywords,
             tokens,
-            scope_delta: syntax.scope_delta,
+            scope_changed: syntax.scope_changed,
         }),
         pos,
     ))
diff --git a/src/syntax.rs b/src/syntax.rs
index 309d1c74..34632f9e 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -87,28 +87,33 @@ pub struct CustomSyntax {
     pub parse: Box<FnCustomSyntaxParse>,
     /// Custom syntax implementation function.
     pub func: Shared<FnCustomSyntaxEval>,
-    /// Delta number of variables in the scope.
-    pub scope_delta: isize,
+    /// Any variables added/removed in the scope?
+    pub scope_changed: bool,
 }
 
 impl Engine {
     /// Register a custom syntax with the [`Engine`].
     ///
     /// * `keywords` holds a slice of strings that define the custom syntax.  
-    /// * `new_vars` is the number of new variables declared by this custom syntax, or the number of variables removed (if negative).
+    /// * `scope_changed` specifies variables have been added/removed by this custom syntax.
     /// * `func` is the implementation function.
     ///
-    /// # Notes
+    /// # Caveat - Do not change beyond block scope
     ///
-    /// If `new_vars` is positive, then a number of new variables are expected to be pushed into the
-    /// current [`Scope`][crate::Scope].
+    /// If `scope_changed` is `true`, then the current [`Scope`][crate::Scope] is assumed to be
+    /// modified by this custom syntax.
     ///
-    /// If `new_vars` is negative, then it is expected that only the top `new_vars` variables in the
-    /// current [`Scope`][crate::Scope] will be _popped_.  Do not randomly remove variables.
+    /// Adding new variables and/or removing variables are allowed. Simply modifying the values of
+    /// variables does NOT count, so `false` should be passed in this case.
+    ///
+    /// However, only variables declared within the current _block scope_ should be touched,
+    /// since they all go away at the end of the block.
+    ///
+    /// Variables in parent blocks should be left untouched as they persist beyond the current block.
     pub fn register_custom_syntax<S: AsRef<str> + Into<Identifier>>(
         &mut self,
         keywords: &[S],
-        new_vars: isize,
+        scope_changed: bool,
         func: impl Fn(&mut EvalContext, &[Expression]) -> RhaiResult + SendSync + 'static,
     ) -> Result<&mut Self, ParseError> {
         let keywords = keywords.as_ref();
@@ -203,7 +208,7 @@ impl Engine {
                     Ok(Some(segments[stream.len()].clone()))
                 }
             },
-            new_vars,
+            scope_changed,
             func,
         );
 
@@ -215,7 +220,7 @@ impl Engine {
     ///
     /// This function is very low level.
     ///
-    /// * `new_vars` is the number of new variables declared by this custom syntax, or the number of variables removed (if negative).
+    /// * `scope_changed` specifies variables have been added/removed by this custom syntax.
     /// * `parse` is the parsing function.
     /// * `func` is the implementation function.
     ///
@@ -227,7 +232,7 @@ impl Engine {
         parse: impl Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError>
             + SendSync
             + 'static,
-        new_vars: isize,
+        scope_changed: bool,
         func: impl Fn(&mut EvalContext, &[Expression]) -> RhaiResult + SendSync + 'static,
     ) -> &mut Self {
         self.custom_syntax.insert(
@@ -235,7 +240,7 @@ impl Engine {
             Box::new(CustomSyntax {
                 parse: Box::new(parse),
                 func: (Box::new(func) as Box<FnCustomSyntaxEval>).into(),
-                scope_delta: new_vars,
+                scope_changed,
             }),
         );
         self
diff --git a/tests/syntax.rs b/tests/syntax.rs
index 5dd03d9a..2fa1dc0c 100644
--- a/tests/syntax.rs
+++ b/tests/syntax.rs
@@ -19,15 +19,15 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
 
     engine.register_custom_syntax(
         &[
-            "exec", "|", "$ident$", "|", "->", "$block$", "while", "$expr$",
+            "exec", "[", "$ident$", "]", "->", "$block$", "while", "$expr$",
         ],
-        1,
+        true,
         |context, inputs| {
             let var_name = inputs[0].get_variable_name().unwrap().to_string();
             let stmt = inputs.get(1).unwrap();
             let condition = inputs.get(2).unwrap();
 
-            context.scope_mut().push(var_name, 0 as INT);
+            context.scope_mut().push(var_name.clone(), 0 as INT);
 
             let mut count: INT = 0;
 
@@ -35,6 +35,10 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
                 context.eval_expression_tree(stmt)?;
                 count += 1;
 
+                context
+                    .scope_mut()
+                    .push(format!("{}{}", var_name, count), count);
+
                 let stop = !context
                     .eval_expression_tree(condition)?
                     .as_bool()
@@ -59,7 +63,7 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
         engine.eval::<INT>(
             "
                 let x = 0;
-                let foo = (exec |x| -> { x += 2 } while x < 42) * 10;
+                let foo = (exec [x] -> { x += 2 } while x < 42) * 10;
                 foo
             "
         )?,
@@ -69,7 +73,7 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
         engine.eval::<INT>(
             "
                 let x = 0;
-                exec |x| -> { x += 1 } while x < 42;
+                exec [x] -> { x += 1 } while x < 42;
                 x
             "
         )?,
@@ -78,17 +82,27 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
     assert_eq!(
         engine.eval::<INT>(
             "
-                exec |x| -> { x += 1 } while x < 42;
+                exec [x] -> { x += 1 } while x < 42;
                 x
             "
         )?,
         42
     );
+    assert_eq!(
+        engine.eval::<INT>(
+            "
+                let foo = 123;
+                exec [x] -> { x += 1 } while x < 42;
+                foo + x + x1 + x2 + x3
+            "
+        )?,
+        171
+    );
 
     // The first symbol must be an identifier
     assert_eq!(
         *engine
-            .register_custom_syntax(&["!"], 0, |_, _| Ok(Dynamic::UNIT))
+            .register_custom_syntax(&["!"], false, |_, _| Ok(Dynamic::UNIT))
             .expect_err("should error")
             .0,
         ParseErrorType::BadInput(LexError::ImproperSymbol(
@@ -121,7 +135,7 @@ fn test_custom_syntax_raw() -> Result<(), Box<EvalAltResult>> {
             },
             _ => unreachable!(),
         },
-        1,
+        true,
         |context, inputs| {
             context.scope_mut().push("foo", 999 as INT);