diff --git a/Cargo.lock b/Cargo.lock
index e9b84e2a181..5a803c54543 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -347,6 +347,7 @@ dependencies = [
  "clap",
  "color-eyre",
  "colored",
+ "cow-utils",
  "dhat",
  "jemallocator",
  "phf",
@@ -508,6 +509,7 @@ dependencies = [
 name = "boa_macros"
 version = "0.20.0"
 dependencies = [
+ "cow-utils",
  "proc-macro2",
  "quote",
  "syn",
@@ -587,6 +589,7 @@ dependencies = [
  "color-eyre",
  "colored",
  "comfy-table",
+ "cow-utils",
  "phf",
  "rayon",
  "rustc-hash 2.1.0",
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index d4d5bf6c0be..ed4987adddb 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -25,6 +25,7 @@ phf = { workspace = true, features = ["macros"] }
 pollster.workspace = true
 dhat = { workspace = true, optional = true }
 color-eyre.workspace = true
+cow-utils.workspace = true
 
 [features]
 default = ["boa_engine/annex-b", "boa_engine/experimental", "boa_engine/intl_bundled"]
diff --git a/cli/src/debug/function.rs b/cli/src/debug/function.rs
index 8337ba708b7..8edc167d16b 100644
--- a/cli/src/debug/function.rs
+++ b/cli/src/debug/function.rs
@@ -5,6 +5,7 @@ use boa_engine::{
     vm::flowgraph::{Direction, Graph},
     Context, JsArgs, JsNativeError, JsObject, JsResult, JsValue, NativeFunction,
 };
+use cow_utils::CowUtils;
 
 use crate::FlowgraphFormat;
 
@@ -14,7 +15,7 @@ fn flowgraph_parse_format_option(value: &JsValue) -> JsResult<FlowgraphFormat> {
     }
 
     if let Some(string) = value.as_string() {
-        return match string.to_std_string_escaped().to_lowercase().as_str() {
+        return match string.to_std_string_escaped().cow_to_lowercase().as_ref() {
             "mermaid" => Ok(FlowgraphFormat::Mermaid),
             "graphviz" => Ok(FlowgraphFormat::Graphviz),
             format => Err(JsNativeError::typ()
@@ -34,7 +35,7 @@ fn flowgraph_parse_direction_option(value: &JsValue) -> JsResult<Direction> {
     }
 
     if let Some(string) = value.as_string() {
-        return match string.to_std_string_escaped().to_lowercase().as_str() {
+        return match string.to_std_string_escaped().cow_to_lowercase().as_ref() {
             "leftright" | "lr" => Ok(Direction::LeftToRight),
             "rightleft" | "rl" => Ok(Direction::RightToLeft),
             "topbottom" | "tb" => Ok(Direction::TopToBottom),
diff --git a/clippy.toml b/clippy.toml
index 59ef99e1fe1..5b248b4ae4b 100644
--- a/clippy.toml
+++ b/clippy.toml
@@ -1,5 +1,10 @@
 doc-valid-idents = ['ECMAScript', 'JavaScript', 'SpiderMonkey', 'GitHub']
 allow-print-in-tests = true
 disallowed-methods = [
+  { path = "str::to_ascii_lowercase", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_to_ascii_lowercase` instead." },
   { path = "str::to_ascii_uppercase", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_to_ascii_uppercase` instead." },
+  { path = "str::to_lowercase", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_to_lowercase` instead." },
+  { path = "str::to_uppercase", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_to_uppercase` instead." },
+  { path = "str::replace", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_replace` instead." },
+  { path = "str::replacen", reason = "To avoid memory allocation, use `cow_utils::CowUtils::cow_replacen` instead." },
 ]
diff --git a/core/engine/src/builtins/number/globals.rs b/core/engine/src/builtins/number/globals.rs
index c9ef2a4b2b8..1418e32837f 100644
--- a/core/engine/src/builtins/number/globals.rs
+++ b/core/engine/src/builtins/number/globals.rs
@@ -8,6 +8,7 @@ use crate::{
 };
 
 use boa_macros::js_str;
+use cow_utils::CowUtils;
 
 /// Builtin javascript 'isFinite(number)' function.
 ///
@@ -304,8 +305,8 @@ pub(crate) fn parse_float(
         // TODO: parse float with optimal utf16 algorithm
         let input_string = val.to_string(context)?.to_std_string_escaped();
         let s = input_string.trim_start_matches(is_trimmable_whitespace);
-        let s_prefix_lower = s.chars().take(4).collect::<String>().to_ascii_lowercase();
-
+        let s_prefix = s.chars().take(4).collect::<String>();
+        let s_prefix_lower = s_prefix.cow_to_ascii_lowercase();
         // TODO: write our own lexer to match syntax StrDecimalLiteral
         if s.starts_with("Infinity") || s.starts_with("+Infinity") {
             Ok(JsValue::new(f64::INFINITY))
diff --git a/core/engine/src/builtins/number/mod.rs b/core/engine/src/builtins/number/mod.rs
index 2611800feba..b9b0d86e019 100644
--- a/core/engine/src/builtins/number/mod.rs
+++ b/core/engine/src/builtins/number/mod.rs
@@ -26,6 +26,7 @@ use crate::{
     Context, JsArgs, JsResult, JsString,
 };
 use boa_profiler::Profiler;
+use cow_utils::CowUtils;
 use num_traits::float::FloatCore;
 
 mod globals;
@@ -916,7 +917,7 @@ impl Number {
 /// Helper function that formats a float as a ES6-style exponential number string.
 fn f64_to_exponential(n: f64) -> JsString {
     js_string!(match n.abs() {
-        x if x >= 1.0 || x == 0.0 => format!("{n:e}").replace('e', "e+"),
+        x if x >= 1.0 || x == 0.0 => format!("{n:e}").cow_replace('e', "e+").to_string(),
         _ => format!("{n:e}"),
     })
 }
diff --git a/core/engine/src/builtins/string/mod.rs b/core/engine/src/builtins/string/mod.rs
index 77d75771af5..96113d4580b 100644
--- a/core/engine/src/builtins/string/mod.rs
+++ b/core/engine/src/builtins/string/mod.rs
@@ -25,6 +25,7 @@ use crate::{
 use boa_macros::utf16;
 
 use boa_profiler::Profiler;
+use cow_utils::CowUtils;
 use icu_normalizer::{ComposingNormalizer, DecomposingNormalizer};
 use std::cmp::{max, min};
 
@@ -1727,9 +1728,9 @@ impl String {
         // the Unicode Default Case Conversion algorithm.
         let text = string.map_valid_segments(|s| {
             if UPPER {
-                s.to_uppercase()
+                s.cow_to_uppercase().to_string()
             } else {
-                s.to_lowercase()
+                s.cow_to_lowercase().to_string()
             }
         });
 
diff --git a/core/engine/src/builtins/temporal/zoneddatetime/mod.rs b/core/engine/src/builtins/temporal/zoneddatetime/mod.rs
index 5d989d086f6..e9979fc2d0e 100644
--- a/core/engine/src/builtins/temporal/zoneddatetime/mod.rs
+++ b/core/engine/src/builtins/temporal/zoneddatetime/mod.rs
@@ -17,6 +17,7 @@ use crate::{
 };
 use boa_gc::{Finalize, Trace};
 use boa_profiler::Profiler;
+use cow_utils::CowUtils;
 use num_traits::ToPrimitive;
 use temporal_rs::{
     options::{ArithmeticOverflow, Disambiguation, OffsetDisambiguation},
@@ -453,7 +454,7 @@ impl ZonedDateTime {
 
         let era = zdt.inner.era_with_provider(context.tz_provider())?;
         Ok(era
-            .map(|tinystr| JsString::from(tinystr.to_lowercase()))
+            .map(|tinystr| JsString::from(tinystr.cow_to_lowercase().to_string()))
             .into_or_undefined())
     }
 
diff --git a/core/macros/Cargo.toml b/core/macros/Cargo.toml
index 6f43d7dd8cb..81fc6c29872 100644
--- a/core/macros/Cargo.toml
+++ b/core/macros/Cargo.toml
@@ -12,6 +12,7 @@ rust-version.workspace = true
 proc-macro = true
 
 [dependencies]
+cow-utils.workspace = true
 quote.workspace = true
 syn = { workspace = true, features = ["full"] }
 proc-macro2.workspace = true
diff --git a/core/macros/src/lib.rs b/core/macros/src/lib.rs
index b5247c4b59b..9f22e884a4f 100644
--- a/core/macros/src/lib.rs
+++ b/core/macros/src/lib.rs
@@ -6,6 +6,7 @@
 )]
 #![cfg_attr(not(test), forbid(clippy::unwrap_used))]
 
+use cow_utils::CowUtils;
 use proc_macro::TokenStream;
 use proc_macro2::Literal;
 use quote::{quote, ToTokens};
@@ -65,14 +66,14 @@ impl Parse for Static {
                 let ident = if let Some(ident) = ident {
                     syn::parse2::<Ident>(ident.into_token_stream())?
                 } else {
-                    Ident::new(&literal.value().to_uppercase(), literal.span())
+                    Ident::new(&literal.value().cow_to_uppercase(), literal.span())
                 };
 
                 Ok(Self { literal, ident })
             }
             Expr::Lit(expr) => match expr.lit {
                 Lit::Str(str) => Ok(Self {
-                    ident: Ident::new(&str.value().to_uppercase(), str.span()),
+                    ident: Ident::new(&str.value().cow_to_uppercase(), str.span()),
                     literal: str,
                 }),
                 _ => Err(syn::Error::new_spanned(
@@ -108,9 +109,9 @@ pub fn static_syms(input: TokenStream) -> TokenStream {
             "Symbol for the \"{}\" string.",
             lit.literal
                 .value()
-                .replace('<', r"\<")
-                .replace('>', r"\>")
-                .replace('*', r"\*")
+                .cow_replace('<', r"\<")
+                .cow_replace('>', r"\>")
+                .cow_replace('*', r"\*")
         );
         let ident = &lit.ident;
         idx += 1;
diff --git a/tests/tester/Cargo.toml b/tests/tester/Cargo.toml
index f2e5f165df3..ce6f3b9023b 100644
--- a/tests/tester/Cargo.toml
+++ b/tests/tester/Cargo.toml
@@ -29,6 +29,7 @@ phf = { workspace = true, features = ["macros"] }
 comfy-table.workspace = true
 serde_repr.workspace = true
 bus.workspace = true
+cow-utils.workspace = true
 
 [features]
 default = ["boa_engine/intl_bundled", "boa_engine/experimental", "boa_engine/annex-b"]
diff --git a/tests/tester/src/read.rs b/tests/tester/src/read.rs
index 1cc85a3a925..f9af9bd4ce9 100644
--- a/tests/tester/src/read.rs
+++ b/tests/tester/src/read.rs
@@ -11,6 +11,7 @@ use color_eyre::{
     eyre::{OptionExt, WrapErr},
     Result,
 };
+use cow_utils::CowUtils;
 use rustc_hash::{FxBuildHasher, FxHashMap};
 use serde::Deserialize;
 
@@ -247,7 +248,7 @@ fn read_metadata(test: &Path) -> Result<MetaData> {
     let (metadata, _) = metadata
         .split_once("---*/")
         .ok_or_eyre("invalid test metadata")?;
-    let metadata = metadata.replace('\r', "\n");
+    let metadata = metadata.cow_replace('\r', "\n");
 
     serde_yaml::from_str(&metadata).map_err(Into::into)
 }