Add RegEx substitution testcase and fix relevant docs

(cherry picked from commit a31bbb4bd920e28cbb2c737a785e91da5e7a599f)

main/tests/test_string.cpp

@@ -33,6 +33,7 @@
 //#include "core/math/math_funcs.h"
 #include "core/io/ip_address.h"
 #include "core/os/os.h"
+#include "modules/regex/regex.h"
 #include <stdio.h>
 
 #include "test_string.h"
@@ -429,9 +430,20 @@ bool test_25() {
 
 bool test_26() {
 
-	//TODO: Do replacement RegEx test
-	return true;
-};
+	OS::get_singleton()->print("\n\nTest 26: RegEx substitution\n");
+
+	String s = "Double all the vowels.";
+
+	OS::get_singleton()->print("\tString: %ls\n", s.c_str());
+	OS::get_singleton()->print("\tRepeating instances of 'aeiou' once\n");
+
+	RegEx re("(?<vowel>[aeiou])");
+	s = re.sub(s, "$0$vowel", true);
+
+	OS::get_singleton()->print("\tResult: %ls\n", s.c_str());
+
+	return (s == "Doouublee aall thee vooweels.");
+}
 
 struct test_27_data {
 	char const *data;

modules/regex/doc_classes/RegEx.xml

@@ -123,7 +123,7 @@
 			<argument index="4" name="end" type="int" default="-1">
 			</argument>
 			<description>
-				Searches the text for the compiled pattern and replaces it with the specified string. Escapes and backreferences such as [code]\1[/code] and [code]\g&lt;name&gt;[/code] expanded and resolved. By default only the first instance is replaced but it can be changed for all instances (global replacement). The region to search within can be specified without modifying where the start and end anchor would be.
+				Searches the text for the compiled pattern and replaces it with the specified string. Escapes and backreferences such as [code]$1[/code] and [code]$name[/code] are expanded and resolved. By default only the first instance is replaced but it can be changed for all instances (global replacement). The region to search within can be specified without modifying where the start and end anchor would be.
 			</description>
 		</method>
 	</methods>