doc: add doc for Substring

README.md

@@ -550,6 +550,8 @@ import "github.com/duke-git/lancet/v2/strutil"
 - **<big>SplitEx</big>** : split a given string which can control the result slice contains empty string or not.
 [[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil.md#SplitEx)]
 [[play](https://go.dev/play/p/Us-ySSbWh-3)]
+- **<big>Substring</big>** : returns a substring of the specified length starting at the specified offset position.
+[[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil.md#Substring)]
 - **<big>Wrap</big>** : wrap a string with given string.
 [[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil.md#Wrap)]
 [[play](https://go.dev/play/p/KoZOlZDDt9y)]

README_zh-CN.md

@@ -546,6 +546,8 @@ import "github.com/duke-git/lancet/v2/strutil"
 - **<big>SplitEx</big>** : 拆分给定的字符串可以控制结果切片是否包含空字符串。
 [[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil_zh-CN.md#SplitEx)]
 [[play](https://go.dev/play/p/Us-ySSbWh-3)]
+- **<big>Substring</big>** : 根据指定的位置和长度截取子字符串。
+[[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil.md#Substring)]
 - **<big>Wrap</big>** : 用给定字符包裹传入的字符串
 [[doc](https://github.com/duke-git/lancet/blob/main/docs/strutil_zh-CN.md#Wrap)]
 [[play](https://go.dev/play/p/KoZOlZDDt9y)]

docs/strutil.md

@@ -37,6 +37,7 @@ import (
 - [SnakeCase](#SnakeCase)
 - [UpperSnakeCase](#UpperSnakeCase)
 - [SplitEx](#SplitEx)
+- [Substring](#Substring)
 - [Wrap](#Wrap)
 - [Unwrap](#Unwrap)
   
@@ -631,6 +632,42 @@ func main() {
 
 
 
+### <span id="Substring">Substring</span>
+<p>Returns a substring of the specified length starting at the specified offset position.</p>
+
+<b>Signature:</b>
+
+```go
+func Substring(s string, offset int, length uint) string
+```
+<b>Example:</b>
+
+```go
+import (
+	"fmt"
+	"github.com/duke-git/lancet/v2/strutil"
+)
+
+func main() {
+	result1 := strutil.Substring("abcde", 1, 3)
+	fmt.Println(result1) //bcd
+
+	result2 := strutil.Substring("abcde", 1, 5)
+	fmt.Println(result2) //bcde
+
+	result3 := strutil.Substring("abcde", -1, 3)
+	fmt.Println(result3) //e
+
+	result4 := strutil.Substring("abcde", -2, 2)
+	fmt.Println(result4) //de
+
+	result5 := strutil.Substring("abcde", -2, 3)
+	fmt.Println(result5) //de
+
+	result6 := strutil.Substring("你好，欢迎你", 0, 2)
+	fmt.Println(result6) //你好
+}
+```
 
 ### <span id="Wrap">Wrap</span>
 <p>Wrap a string with given string.</p>

docs/strutil_zh-CN.md

@@ -37,6 +37,7 @@ import (
 - [SnakeCase](#SnakeCase)
 - [UpperSnakeCase](#UpperSnakeCase)
 - [SplitEx](#SplitEx)
+- [Substring](#Substring)
 - [Wrap](#Wrap)
 - [Unwrap](#Unwrap)
   
@@ -600,6 +601,42 @@ func main() {
 }
 ```
 
+### <span id="Substring">Substring</span>
+<p>根据指定的位置和长度截取子字符串</p>
+
+<b>函数签名:</b>
+
+```go
+func Substring(s string, offset int, length uint) string
+```
+<b>例子:</b>
+
+```go
+import (
+	"fmt"
+	"github.com/duke-git/lancet/v2/strutil"
+)
+
+func main() {
+	result1 := strutil.Substring("abcde", 1, 3)
+	fmt.Println(result1) //bcd
+
+	result2 := strutil.Substring("abcde", 1, 5)
+	fmt.Println(result2) //bcde
+
+	result3 := strutil.Substring("abcde", -1, 3)
+	fmt.Println(result3) //e
+
+	result4 := strutil.Substring("abcde", -2, 2)
+	fmt.Println(result4) //de
+
+	result5 := strutil.Substring("abcde", -2, 3)
+	fmt.Println(result5) //de
+
+	result6 := strutil.Substring("你好，欢迎你", 0, 2)
+	fmt.Println(result6) //你好
+}
+```
 
 
 ### <span id="Wrap">Wrap</span>