diff --git a/lslopt/lslbasefuncs.py b/lslopt/lslbasefuncs.py
index 971e476..ae7a773 100644
--- a/lslopt/lslbasefuncs.py
+++ b/lslopt/lslbasefuncs.py
@@ -1818,6 +1818,31 @@ def llPow(base, exp):
     except ValueError:  # should happen only with negative base and noninteger exponent
         return Indet
 
+def llReplaceSubString(source, search, replace, count):
+    source = fs(source)
+    search = fs(search)
+    replace = fs(replace)
+    count = fi(count)
+
+    if not search:
+        return source
+    if count == 0:
+        return source.replace(search, replace)
+    if count > 0:
+        return source.replace(search, replace, count)
+    # Replace *last* occurrences of "search" in "source"
+    slen = len(search)
+    i = len(source)
+    while True:
+        i = source.rfind(search, 0, i)
+        if i < 0:
+            break
+        source = source[:i] + replace + source[i + slen:]
+        count += 1
+        if count == 0:
+            break
+    return source
+
 def llRot2Angle(r):
     r = q2f(r)
     # Used by llAngleBetween.
diff --git a/unit_tests/expr.suite/string-funcs.lsl b/unit_tests/expr.suite/string-funcs.lsl
index 156d2ab..11f701d 100644
--- a/unit_tests/expr.suite/string-funcs.lsl
+++ b/unit_tests/expr.suite/string-funcs.lsl
@@ -197,4 +197,33 @@
 , llSubStringIndex("a", "blah")
 , llStringTrim(" a\n	
", STRING_TRIM)
 , llStringTrim("", STRING_TRIM)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", -4)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", -3)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", -2)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", -1)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", 0)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", 1)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", 2)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", 3)
+, llReplaceSubString("cabadabaeaba", "aba", "faba", 4)
+, llReplaceSubString("cabadabaeaba", "eba", "feba", -4)
+, llReplaceSubString("cabadabaeaba", "eba", "feba", -1)
+, llReplaceSubString("cabadabaeaba", "eba", "feba", 0)
+, llReplaceSubString("cabadabaeaba", "eba", "feba", 1)
+, llReplaceSubString("cabadabaeaba", "eba", "feba", 4)
+, llReplaceSubString("cabadabaeaba", "", "faba", -20)
+, llReplaceSubString("cabadabaeaba", "", "faba", -1)
+, llReplaceSubString("cabadabaeaba", "", "faba", 0)
+, llReplaceSubString("cabadabaeaba", "", "faba", 1)
+, llReplaceSubString("cabadabaeaba", "", "faba", 20)
+, llReplaceSubString("", "", "faba", -2)
+, llReplaceSubString("", "", "faba", -1)
+, llReplaceSubString("", "", "faba", 0)
+, llReplaceSubString("", "", "faba", 1)
+, llReplaceSubString("", "", "faba", 2)
+, llReplaceSubString("", "a", "ba", -2)
+, llReplaceSubString("", "a", "ba", -1)
+, llReplaceSubString("", "a", "ba", 0)
+, llReplaceSubString("", "a", "ba", 1)
+, llReplaceSubString("", "a", "ba", 2)
 ]
diff --git a/unit_tests/expr.suite/string-funcs.out b/unit_tests/expr.suite/string-funcs.out
index 09466a3..128c8fc 100644
--- a/unit_tests/expr.suite/string-funcs.out
+++ b/unit_tests/expr.suite/string-funcs.out
@@ -197,4 +197,33 @@
 , -1
 , "a"
 , ""
+, "cfabadfabaefaba"
+, "cfabadfabaefaba"
+, "cabadfabaefaba"
+, "cabadabaefaba"
+, "cfabadfabaefaba"
+, "cfabadabaeaba"
+, "cfabadfabaeaba"
+, "cfabadfabaefaba"
+, "cfabadfabaefaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, "cabadabaeaba"
+, ""
+, ""
+, ""
+, ""
+, ""
+, ""
+, ""
+, ""
+, ""
+, ""
 ]
\ No newline at end of file