diff --git a/cores/esp8266/WString.cpp b/cores/esp8266/WString.cpp
index 6e6dc94beb..8184224957 100644
--- a/cores/esp8266/WString.cpp
+++ b/cores/esp8266/WString.cpp
@@ -273,6 +273,13 @@ String &String::operator =(const __FlashStringHelper *pstr) {
     return *this;
 }
 
+String &String::operator =(char c) {
+    char buffer[2] { c, '\0' };
+    *this = buffer;
+    return *this;
+}
+
+
 /*********************************************/
 /*  concat                                   */
 /*********************************************/
@@ -500,6 +507,10 @@ bool String::equals(const char *cstr) const {
     return strcmp(buffer(), cstr) == 0;
 }
 
+bool String::equals(const __FlashStringHelper *s) const {
+    return equals(String(s));
+}
+
 bool String::operator<(const String &rhs) const {
     return compareTo(rhs) < 0;
 }
@@ -532,6 +543,10 @@ bool String::equalsIgnoreCase(const String &s2) const {
     return true;
 }
 
+bool String::equalsIgnoreCase(const __FlashStringHelper *s) const {
+    return equalsIgnoreCase(String(s));
+}
+
 unsigned char String::equalsConstantTime(const String &s2) const {
     // To avoid possible time-based attacks present function
     // compares given strings in a constant time.
@@ -565,18 +580,37 @@ bool String::startsWith(const String &s2) const {
     return startsWith(s2, 0);
 }
 
+bool String::startsWith(const char *prefix) const {
+    return this->startsWith(String(prefix));
+}
+bool String::startsWith(const __FlashStringHelper *prefix) const {
+    return this->startsWith(String(prefix));
+}
+
 bool String::startsWith(const String &s2, unsigned int offset) const {
     if (offset > (unsigned)(len() - s2.len()) || !buffer() || !s2.buffer())
         return false;
     return strncmp(&buffer()[offset], s2.buffer(), s2.len()) == 0;
 }
 
+bool String::startsWith(const __FlashStringHelper *prefix, unsigned int offset) const {
+    return startsWith(String(prefix), offset);
+}
+
 bool String::endsWith(const String &s2) const {
     if (len() < s2.len() || !buffer() || !s2.buffer())
         return false;
     return strcmp(&buffer()[len() - s2.len()], s2.buffer()) == 0;
 }
 
+bool String::endsWith(const char *suffix) const {
+    return this->endsWith(String(suffix));
+}
+bool String::endsWith(const __FlashStringHelper *suffix) const {
+    return this->endsWith(String(suffix));
+}
+
+
 /*********************************************/
 /*  Character Access                         */
 /*********************************************/
@@ -678,6 +712,15 @@ int String::lastIndexOf(const String &s2, unsigned int fromIndex) const {
     return found;
 }
 
+int String::lastIndexOf(const __FlashStringHelper *str) const {
+    return lastIndexOf(String(str));
+}
+
+int String::lastIndexOf(const __FlashStringHelper *str, unsigned int fromIndex) const {
+    return lastIndexOf(String(str), fromIndex);
+}
+
+
 String String::substring(unsigned int left, unsigned int right) const {
     if (left > right) {
         unsigned int temp = right;
@@ -756,6 +799,24 @@ void String::replace(const String &find, const String &replace) {
     }
 }
 
+
+void String::replace(const char *find, const String &replace) {
+    this->replace(String(find), replace);
+}
+void String::replace(const __FlashStringHelper *find, const String &replace) {
+    this->replace(String(find), replace);
+}
+void String::replace(const char *find, const char *replace) {
+    this->replace(String(find), String(replace));
+}
+void String::replace(const __FlashStringHelper *find, const char *replace) {
+    this->replace(String(find), String(replace));
+}
+void String::replace(const __FlashStringHelper *find, const __FlashStringHelper *replace) {
+    this->replace(String(find), String(replace));
+}
+
+
 void String::remove(unsigned int index, unsigned int count) {
     if (index >= len()) {
         return;
diff --git a/cores/esp8266/WString.h b/cores/esp8266/WString.h
index 37bb3becb6..ad35a0cb4a 100644
--- a/cores/esp8266/WString.h
+++ b/cores/esp8266/WString.h
@@ -101,11 +101,7 @@ class String {
         String &operator =(const char *cstr);
         String &operator =(const __FlashStringHelper *str);
         String &operator =(String &&rval) noexcept;
-        String &operator =(char c) {
-            char buffer[2] { c, '\0' };
-            *this = buffer;
-            return *this;
-        }
+        String &operator =(char c);
 
         // concatenate (works w/ built-in types)
 
@@ -142,39 +138,40 @@ class String {
         int compareTo(const String &s) const;
         bool equals(const String &s) const;
         bool equals(const char *cstr) const;
+        bool equals(const __FlashStringHelper *s) const;
         bool operator ==(const String &rhs) const {
             return equals(rhs);
         }
         bool operator ==(const char *cstr) const {
             return equals(cstr);
         }
+        bool operator ==(const __FlashStringHelper *rhs) const {
+            return equals(rhs);
+        }
         bool operator !=(const String &rhs) const {
             return !equals(rhs);
         }
         bool operator !=(const char *cstr) const {
             return !equals(cstr);
         }
+        bool operator !=(const __FlashStringHelper *rhs) const {
+            return !equals(rhs);
+        }
         bool operator <(const String &rhs) const;
         bool operator >(const String &rhs) const;
         bool operator <=(const String &rhs) const;
         bool operator >=(const String &rhs) const;
         bool equalsIgnoreCase(const String &s) const;
+        bool equalsIgnoreCase(const __FlashStringHelper *s) const;
         unsigned char equalsConstantTime(const String &s) const;
         bool startsWith(const String &prefix) const;
-        bool startsWith(const char *prefix) const {
-            return this->startsWith(String(prefix));
-        }
-        bool startsWith(const __FlashStringHelper *prefix) const {
-            return this->startsWith(String(prefix));
-        }
+        bool startsWith(const char *prefix) const;
+        bool startsWith(const __FlashStringHelper *prefix) const;
         bool startsWith(const String &prefix, unsigned int offset) const;
+        bool startsWith(const __FlashStringHelper *prefix, unsigned int offset) const;
         bool endsWith(const String &suffix) const;
-        bool endsWith(const char *suffix) const {
-            return this->endsWith(String(suffix));
-        }
-        bool endsWith(const __FlashStringHelper *suffix) const {
-            return this->endsWith(String(suffix));
-        }
+        bool endsWith(const char *suffix) const;
+        bool endsWith(const __FlashStringHelper *suffix) const;
 
         // character access
         char charAt(unsigned int index) const {
@@ -204,6 +201,8 @@ class String {
         int lastIndexOf(char ch, unsigned int fromIndex) const;
         int lastIndexOf(const String &str) const;
         int lastIndexOf(const String &str, unsigned int fromIndex) const;
+        int lastIndexOf(const __FlashStringHelper *str) const;
+        int lastIndexOf(const __FlashStringHelper *str, unsigned int fromIndex) const;
         String substring(unsigned int beginIndex) const {
             return substring(beginIndex, len());
         }
@@ -212,21 +211,12 @@ class String {
         // modification
         void replace(char find, char replace);
         void replace(const String &find, const String &replace);
-        void replace(const char *find, const String &replace) {
-            this->replace(String(find), replace);
-        }
-        void replace(const __FlashStringHelper *find, const String &replace) {
-            this->replace(String(find), replace);
-        }
-        void replace(const char *find, const char *replace) {
-            this->replace(String(find), String(replace));
-        }
-        void replace(const __FlashStringHelper *find, const char *replace) {
-            this->replace(String(find), String(replace));
-        }
-        void replace(const __FlashStringHelper *find, const __FlashStringHelper *replace) {
-            this->replace(String(find), String(replace));
-        }
+        void replace(const char *find, const String &replace);
+        void replace(const __FlashStringHelper *find, const String &replace);
+        void replace(const char *find, const char *replace);
+        void replace(const __FlashStringHelper *find, const char *replace);
+        void replace(const __FlashStringHelper *find, const __FlashStringHelper *replace);
+
         // Pass the biggest integer if the count is not specified.
         // The remove method below will take care of truncating it at the end of the string.
         void remove(unsigned int index, unsigned int count = (unsigned int)-1);