added sub_str

README.md

@@ -38,7 +38,8 @@ Some modules may depend on other modules. For example, the `polyline2d` module d
 
 - Functon
 	- [rotate_p](https://openhome.cc/eGossip/OpenSCAD/lib-rotate_p.html)
-
+	- [sub_str](https://openhome.cc/eGossip/OpenSCAD/lib-sub_str.html)
+	
 - Path
     - [circle_path](https://openhome.cc/eGossip/OpenSCAD/lib-circle_path.html)
     - [bezier](https://openhome.cc/eGossip/OpenSCAD/lib-bezier.html)

docs/lib-sub_str.md

@@ -0,0 +1,14 @@
+# sub_str
+
+Returns a new string that is a substring of the given string.
+
+## Parameters
+
+- `t` : The original string.
+- `begin` : The beginning index, inclusive.
+- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of this string.
+
+## Examples
+
+	echo(sub_str("helloworld", 0, 5)); // ECHO: "hello"
+	echo(sub_str("helloworld", 5));    // ECHO: "world"

src/sub_str.scad

@@ -0,0 +1,17 @@
+/**
+* sub_str.scad
+*
+* Returns a new string that is a substring of the given string.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-sub_str.html
+*
+**/ 
+
+function sub_str(t, begin, end, result = "") =
+    end == undef ? sub_str(t, begin, len(t)) : (
+        begin == end ? result : sub_str(t, begin + 1, end, str(result, t[begin]))
+    );
+