golang截取字符串
``` golang
//截取字符串 start 起点下标 length 需要截取的长度
func Substr(str string, start int, length int) string {
rs := []rune(str)
rl := len(rs)
end := 0
if start < 0 {
start = rl - 1 + start
}
end = start + length
if start > end {
start, end = end, start
}
if start < 0 {
start = 0
}
if start > rl {
start = rl
}
if end < 0 {
end = 0
}
if end > rl {
end = rl
}
return string(rs[start:end])
}
//截取字符串 start 起点下标 end 终点下标(不包括)
func Substr2(str string, start int, end int) string {
rs := []rune(str)
length := len(rs)
if start < 0 || start > length {
panic("start is wrong")
}
if end < 0 || end > length {
panic("end is wrong")
}
return string(rs[start:end])
}
```