unicode

Before You Start

• Applies to: Go 1.21+ (examples verified with Go 1.22) · Prerequisites: Base: Strings · Read time: about 8 min

In Go language development, string slicing is a high-frequency operation. Special attention needs to be paid to encoding issues, especially when dealing with mixed Chinese and English text. This article systematically organizes four mainstream string slicing methods in Go, analyzing technical points with code examples.

Unicode Safe Processing Method

Achieve character-level safe slicing through []rune conversion:

func main() {
    str := "Hello, 世界!"
    runes := []rune(str)
    
    // Character-level safe slicing
    sub3 := string(runes[7:9])
    fmt.Println("Safe Chinese slicing:", sub3) // Output: 世界
}

Technical Comparison:

Method	Time Complexity	Memory Overhead	Applicable Scenarios
Byte Slice	O(1)	Low	Pure ASCII strings
[]rune Conversion	O(n)	High	Strings containing multi-byte characters

Complete Example

The program below runs as-is: copy it into a file and execute go run main.go, or paste it into the Go Playground.

package main

import (
	"fmt"
	"unicode"
	"unicode/utf8"
)

func main() {
	r := 'G'
	fmt.Println(unicode.IsLetter(r), unicode.IsUpper(r), unicode.ToLower(r))

	cn := '中'
	fmt.Println(unicode.Is(unicode.Han, cn), utf8.RuneLen(cn))

	count := 0
	for _, r := range "a1中!" {
		if unicode.IsLetter(r) {
			count++
		}
	}
	fmt.Println("letters:", count)
}

Expected output:

true true 103
true 3
letters: 2

Common Pitfalls

• len is bytes, not characters. Use utf8.RuneCountInString for character counts.
• Test categories, not code points. unicode.Is(unicode.Han, r) is safer than comparing numeric ranges.
• Case mapping can expand. Some runes uppercase to multiple characters; length is not preserved.
• Convert to []rune before indexing characters. []rune(s)[i] gives the i-th character, not byte.
• utf8 validates boundaries. utf8.DecodeRuneInString is the safe way to step through a string.

Change Log

• 2026-10-14: Added "Before You Start" notes, a complete runnable example with expected output, common pitfalls, and this change log.