diff --git a/include/nlohmann/detail/string_escape.hpp b/include/nlohmann/detail/string_escape.hpp
index 63715cde13..36e76bd0fd 100644
--- a/include/nlohmann/detail/string_escape.hpp
+++ b/include/nlohmann/detail/string_escape.hpp
@@ -15,58 +15,106 @@ namespace detail
 {
 
 /*!
-@brief replace all occurrences of a substring by another string
-
-@param[in,out] s  the string to manipulate; changed so that all
-               occurrences of @a f are replaced with @a t
-@param[in]     f  the substring to replace with @a t
-@param[in]     t  the string to replace @a f
-
-@pre The search string @a f must not be empty. **This precondition is
-enforced with an assertion.**
-
-@since version 2.0.0
-*/
-template<typename StringType>
-inline void replace_substring(StringType& s, const StringType& f,
-                              const StringType& t)
-{
-    JSON_ASSERT(!f.empty());
-    for (auto pos = s.find(f);                // find the first occurrence of f
-            pos != StringType::npos;          // make sure f was found
-            s.replace(pos, f.size(), t),      // replace with t, and
-            pos = s.find(f, pos + t.size()))  // find the next occurrence of f
-    {}
-}
-
-/*!
- * @brief string escaping as described in RFC 6901 (Sect. 4)
+ * @brief Returns a copy of a string escaped as described in RFC 6901 (Sect. 4)
  * @param[in] s string to escape
  * @return    escaped string
  *
- * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
  */
-template<typename StringType>
-inline StringType escape(StringType s)
+template<typename StringType> // [[nodiscard]]
+inline StringType escape(StringType const& s)
 {
-    replace_substring(s, StringType{"~"}, StringType{"~0"});
-    replace_substring(s, StringType{"/"}, StringType{"~1"});
-    return s;
+    using CharT = typename StringType::value_type;
+    StringType res;
+
+    auto esz = s.size();
+    for (auto const ch : s)
+    {
+        if (ch == CharT('~') || ch == CharT('/'))
+        {
+            ++esz;
+        }
+    }
+    if (esz == s.size())
+    {
+        res = s;
+    }
+    else
+    {
+        res.reserve(esz);
+        for (auto const ch : s)   // Yes, this is UTF8-safe
+        {
+            if (ch == CharT('~'))
+            {
+                res.push_back(CharT('~'));
+                res.push_back(CharT{'0'});
+            }
+            else if (ch == CharT('/'))
+            {
+                res.push_back(CharT{'~'});
+                res.push_back(CharT{'1'});
+            }
+            else
+            {
+                res.push_back(ch);
+            }
+        }
+    }
+    return res;
 }
 
 /*!
- * @brief string unescaping as described in RFC 6901 (Sect. 4)
+ * @brief Unescapes a string as described in RFC 6901 (Sect. 4), in-place
  * @param[in] s string to unescape
- * @return    unescaped string
  *
- * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
  */
 template<typename StringType>
 inline void unescape(StringType& s)
 {
-    replace_substring(s, StringType{"~1"}, StringType{"/"});
-    replace_substring(s, StringType{"~0"}, StringType{"~"});
+    using CharT = typename StringType::value_type;
+    auto j = s.begin();
+    while (j != s.end() && *j != CharT('~'))
+    {
+        ++j;
+    }
+    auto i = j;
+    while (i != s.end())
+    {
+        if (*i == CharT('~') && (i + 1) != s.end())
+        {
+            if (*(i + 1) == CharT('0'))
+            {
+                *j++ = CharT('~');
+                ++i;
+            }
+            else if (*(i + 1) == CharT('1'))
+            {
+                *j++ = CharT('/');
+                ++i;
+            } // ... else shouldn't we throw parse_error.108 here?
+        }
+        else
+        {
+            *j++ = *i;
+        }
+        ++i;
+    }
+    s.erase(j, s.end());
+    s.shrink_to_fit();
 }
 
+// Left out, so far we don't use it, so it just lowers test coverage
+// /*!
+// * @brief Out Of Place string unescaping as described in RFC 6901 (Sect. 4)
+// * @param[in] s string to unescape
+// *
+// */
+// template<typename StringType> // [[nodiscard]]
+// inline StringType unescape(StringType const& s)
+// {
+//    StringType res = s;
+//    unescape(res);
+//    return res;
+// }
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index aafeaf5ae5..aceb21bc4c 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -3085,59 +3085,107 @@ namespace detail
 {
 
 /*!
-@brief replace all occurrences of a substring by another string
-
-@param[in,out] s  the string to manipulate; changed so that all
-               occurrences of @a f are replaced with @a t
-@param[in]     f  the substring to replace with @a t
-@param[in]     t  the string to replace @a f
-
-@pre The search string @a f must not be empty. **This precondition is
-enforced with an assertion.**
-
-@since version 2.0.0
-*/
-template<typename StringType>
-inline void replace_substring(StringType& s, const StringType& f,
-                              const StringType& t)
-{
-    JSON_ASSERT(!f.empty());
-    for (auto pos = s.find(f);                // find the first occurrence of f
-            pos != StringType::npos;          // make sure f was found
-            s.replace(pos, f.size(), t),      // replace with t, and
-            pos = s.find(f, pos + t.size()))  // find the next occurrence of f
-    {}
-}
-
-/*!
- * @brief string escaping as described in RFC 6901 (Sect. 4)
+ * @brief Returns a copy of a string escaped as described in RFC 6901 (Sect. 4)
  * @param[in] s string to escape
  * @return    escaped string
  *
- * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
  */
-template<typename StringType>
-inline StringType escape(StringType s)
+template<typename StringType> // [[nodiscard]]
+inline StringType escape(StringType const& s)
 {
-    replace_substring(s, StringType{"~"}, StringType{"~0"});
-    replace_substring(s, StringType{"/"}, StringType{"~1"});
-    return s;
+    using CharT = typename StringType::value_type;
+    StringType res;
+
+    auto esz = s.size();
+    for (auto const ch : s)
+    {
+        if (ch == CharT('~') || ch == CharT('/'))
+        {
+            ++esz;
+        }
+    }
+    if (esz == s.size())
+    {
+        res = s;
+    }
+    else
+    {
+        res.reserve(esz);
+        for (auto const ch : s)   // Yes, this is UTF8-safe
+        {
+            if (ch == CharT('~'))
+            {
+                res.push_back(CharT('~'));
+                res.push_back(CharT{'0'});
+            }
+            else if (ch == CharT('/'))
+            {
+                res.push_back(CharT{'~'});
+                res.push_back(CharT{'1'});
+            }
+            else
+            {
+                res.push_back(ch);
+            }
+        }
+    }
+    return res;
 }
 
 /*!
- * @brief string unescaping as described in RFC 6901 (Sect. 4)
+ * @brief Unescapes a string as described in RFC 6901 (Sect. 4), in-place
  * @param[in] s string to unescape
- * @return    unescaped string
  *
- * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
  */
 template<typename StringType>
 inline void unescape(StringType& s)
 {
-    replace_substring(s, StringType{"~1"}, StringType{"/"});
-    replace_substring(s, StringType{"~0"}, StringType{"~"});
+    using CharT = typename StringType::value_type;
+    auto j = s.begin();
+    while (j != s.end() && *j != CharT('~'))
+    {
+        ++j;
+    }
+    auto i = j;
+    while (i != s.end())
+    {
+        if (*i == CharT('~') && (i + 1) != s.end())
+        {
+            if (*(i + 1) == CharT('0'))
+            {
+                *j++ = CharT('~');
+                ++i;
+            }
+            else if (*(i + 1) == CharT('1'))
+            {
+                *j++ = CharT('/');
+                ++i;
+            } // ... else shouldn't we throw parse_error.108 here?
+        }
+        else
+        {
+            *j++ = *i;
+        }
+        ++i;
+    }
+    s.erase(j, s.end());
+    s.shrink_to_fit();
 }
 
+// Left out, so far we don't use it, so it just lowers test coverage
+// /*!
+// * @brief Out Of Place string unescaping as described in RFC 6901 (Sect. 4)
+// * @param[in] s string to unescape
+// *
+// */
+// template<typename StringType> // [[nodiscard]]
+// inline StringType unescape(StringType const& s)
+// {
+//    StringType res = s;
+//    unescape(res);
+//    return res;
+// }
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
 
diff --git a/tests/src/unit-alt-string.cpp b/tests/src/unit-alt-string.cpp
index fd6047c374..61d74a8984 100644
--- a/tests/src/unit-alt-string.cpp
+++ b/tests/src/unit-alt-string.cpp
@@ -58,6 +58,36 @@ class alt_string
         str_impl.push_back(c);
     }
 
+    void  shrink_to_fit()
+    {
+        str_impl.shrink_to_fit();
+    }
+
+    std::string::iterator begin()
+    {
+        return str_impl.begin();
+    }
+
+    std::string::const_iterator begin() const
+    {
+        return str_impl.begin();
+    }
+
+    std::string::iterator end() noexcept
+    {
+        return str_impl.end();
+    }
+
+    std::string::const_iterator end() const noexcept
+    {
+        return str_impl.end();
+    }
+
+    std::string::iterator erase(std::string::const_iterator first, std::string::const_iterator last )
+    {
+        return str_impl.erase(first, last);
+    }
+
     template <typename op_type>
     bool operator==(const op_type& op) const
     {