diff --git a/src/fox32.pest b/src/fox32.pest
index 19c2079..124ed92 100644
--- a/src/fox32.pest
+++ b/src/fox32.pest
@@ -158,4 +158,4 @@ body_str_chars = {
 }
 
 register = ${ "r" ~ register_num }
-register_num = @{ ASCII_DIGIT+ | "sp" }
+register_num = @{ ASCII_DIGIT+ | "sp" | "esp" }
diff --git a/src/main.rs b/src/main.rs
index 0dec331..eadd173 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -813,8 +813,9 @@ fn parse_operand(mut pair: pest::iterators::Pair<Rule>, is_pointer: bool) -> Ast
                 Rule::register => {
                     let register_num_pair = operand_value_pair.into_inner().next().unwrap();
                     let register_num = if register_num_pair.as_str() == "sp" { 32 }
+                    else if register_num_pair.as_str() == "esp" { 33 }
                     else { register_num_pair.as_str().parse::<u8>().unwrap() };
-                    if register_num > 32 { panic!("register number out of range"); }
+                    if register_num > 33 { panic!("register number out of range"); }
                     if is_pointer {
                         AstNode::RegisterPointer(register_num)
                     } else {