diff --git a/lib/camel_case_keys.js b/lib/camel_case_keys.js
index 6008b910..55e04f3e 100644
--- a/lib/camel_case_keys.js
+++ b/lib/camel_case_keys.js
@@ -2,8 +2,6 @@
 
 const camelCase = require('camel-case');
 
-const rPrefixUnderscore = /^(_+)/;
-
 function getter(key) {
   return function() {
     return this[key];
@@ -16,30 +14,32 @@ function setter(key) {
   };
 }
 
+function toCamelCase(str) {
+  let prefixLength = -1;
+
+  while (str[++prefixLength] === '_');
+
+  if (!prefixLength) {
+    return camelCase(str);
+  }
+  return str.substring(0, prefixLength) + camelCase(str.substring(prefixLength));
+}
+
 function camelCaseKeys(obj) {
   if (typeof obj !== 'object') throw new TypeError('obj must be an object!');
 
   const keys = Object.keys(obj);
+  const { length } = keys;
   const result = {};
 
-  for (let i = 0, len = keys.length; i < len; i++) {
-    const key = keys[i];
-    const value = obj[key];
-    const match = key.match(rPrefixUnderscore);
-    let newKey;
-
-    if (match) {
-      const underscore = match[1];
-      newKey = underscore + camelCase(key.substring(underscore.length));
-    } else {
-      newKey = camelCase(key);
-    }
+  for (let i = 0; i < length; i++) {
+    const oldKey = keys[i];
+    const newKey = toCamelCase(oldKey);
+
+    result[newKey] = obj[oldKey];
 
-    if (newKey === key) {
-      result[key] = value;
-    } else {
-      result[newKey] = value;
-      Object.defineProperty(result, key, {
+    if (newKey !== oldKey) {
+      Object.defineProperty(result, oldKey, {
         get: getter(newKey),
         set: setter(newKey),
         configurable: true,