diff --git a/lib/internal/util/inspect.js b/lib/internal/util/inspect.js
index 659fd7b671e742..434ee6774cb3ef 100644
--- a/lib/internal/util/inspect.js
+++ b/lib/internal/util/inspect.js
@@ -1458,15 +1458,11 @@ function handleMaxCallStackSize(ctx, err, constructorName, indentationLvl) {
 }
 
 function addNumericSeparator(integerString) {
-  let result = '';
-  let i = integerString.length;
-  const start = integerString.startsWith('-') ? 1 : 0;
-  for (; i >= start + 4; i -= 3) {
-    result = `_${integerString.slice(i - 3, i)}${result}`;
-  }
-  return i === integerString.length ?
-    integerString :
-    `${integerString.slice(0, i)}${result}`;
+  return StringPrototypeReplace(
+    integerString,
+    /[0-9](?=(?:[0-9]{3})+(?![0-9]))/g,
+    '$&_'
+  );
 }
 
 function addNumericSeparatorEnd(integerString) {
@@ -1502,7 +1498,7 @@ function formatNumber(fn, number, numericSeparator) {
   return fn(`${
     addNumericSeparator(string)
   }.${
-    addNumericSeparatorEnd(String(number).slice(string.length + 1))
+    StringPrototypeSlice(addNumericSeparatorEnd(String(number), string.length + 1))
   }`, 'number');
 }