From 691ca41c87d9773be0656c5273154b570f5a3d24 Mon Sep 17 00:00:00 2001
From: rulego-team <rulego@outlook.com>
Date: Wed, 6 Aug 2025 10:55:28 +0800
Subject: [PATCH] =?UTF-8?q?fix:=E4=BC=98=E5=8C=96SubstringFunction?=
 =?UTF-8?q?=E8=B4=9F=E6=95=B0=E5=A4=84=E7=90=86=E9=80=BB=E8=BE=91?=
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---
 functions/functions_string.go | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/functions/functions_string.go b/functions/functions_string.go
index a2ce194..dac3f49 100644
--- a/functions/functions_string.go
+++ b/functions/functions_string.go
@@ -295,6 +295,9 @@ func (f *SubstringFunction) Execute(ctx *FunctionContext, args []interface{}) (i
 	}
 
 	strLen := int64(len(str))
+	if start < 0 {
+		start = strLen + start
+	}
 	if start < 0 || start >= strLen {
 		return "", nil
 	}
@@ -308,6 +311,10 @@ func (f *SubstringFunction) Execute(ctx *FunctionContext, args []interface{}) (i
 		return nil, err
 	}
 
+	if length < 0 {
+		return "", nil
+	}
+
 	end := start + length
 	if end > strLen {
 		end = strLen