diff --git a/docs/reference.xml b/docs/reference.xml
index 20ec4435..91aee752 100644
--- a/docs/reference.xml
+++ b/docs/reference.xml
@@ -439,7 +439,7 @@
 					</arguments>
 					<return token="S" description="the substring" />
 					<remarks>
-						<remark>Character indexes begins at 0.</remark>
+						<remark>Character indexes begins at 0. If "start position" is negative, the reference is the end of the string.</remark>
 					</remarks>
 					<examples>
 					<example>
@@ -447,8 +447,8 @@
 						cdefg]]></code>
 					</example>
 					<example>
-						<code><![CDATA[echo (substr abcdefgh 2 3)
-						cde]]></code>
+						<code><![CDATA[echo (substr abcdefgh -3 2)
+						fg]]></code>
 					</example>
 					<example>
 						<code><![CDATA[echo (substr abcdefgh 2)
diff --git a/source/src/command.cpp b/source/src/command.cpp
index f79037d7..1d87f7ab 100644
--- a/source/src/command.cpp
+++ b/source/src/command.cpp
@@ -1266,15 +1266,19 @@ void timestring_()
 
 int millis_() { extern int totalmillis; return totalmillis; }
 void strlen_(char *s) { string r; formatstring(r)("%d", strlen(s)); result(r); }
+
 void substr_(char *fs, char *pa, char *len)
 {
     int ia = atoi(pa);
     int ilen = atoi(len);
-    if(ia>(int)strlen(fs) || ia < 0 || ilen < 0) return;
+    int fslen = (int)strlen(fs);
+    if(ia<0) ia += fslen;
+    if(ia>fslen || ia < 0 || ilen < 0) return;
 
-    if(!ilen) ilen = strlen(fs)-ia;
+    if(!ilen) ilen = fslen-ia;
     result(newstring(fs + ia, ilen));
 }
+
 void l0(int p, int v) { string f; string r; formatstring(f)("%%0%dd", p); formatstring(r)(f, v); result(r); }
 
 COMMANDN(millis, millis_, ARG_IVAL);