Go Strings

Go Strings — Rules and Patterns

1. Runes, bytes, and len

• A Go string is an immutable sequence of bytes, not runes.
• len(s) returns the byte count, not the character/rune count.
• A rune is an alias for int32 and represents a single Unicode code point.
• Under UTF-8 a code point may occupy 1-4 bytes.
• To count runes, use utf8.RuneCountInString(s).

s := "cafe\u0301" // "cafe" + combining accent = 5 runes, but check byte length
fmt.Println(len(s))                    // byte count (likely 6)
fmt.Println(utf8.RuneCountInString(s)) // rune count (5)

2. String iteration

Prefer the range value variable for rune access

When ranging over a string, the index i is the byte offset of the rune, and the value r is the rune itself. Accessing s[i] gives a single byte, not the rune.

Wrong:

for i := range s {
    fmt.Printf("%c", s[i]) // prints individual bytes, corrupts multi-byte runes
}

Correct — iterate over runes:

for i, r := range s {
    fmt.Printf("position %d: %c\n", i, r) // i = byte offset, r = full rune
}

Accessing the i-th rune

If you need the rune at a logical index (not byte offset), convert to []rune first:

r := []rune(s)[4]

This allocates a new slice (O(n)), so avoid it in hot loops. If the string is known to be pure ASCII, rune(s[i]) is safe and allocation-free.

3. Trim functions — TrimRight/TrimLeft vs. TrimSuffix/TrimPrefix

These are commonly confused. The semantics are fundamentally different:

| Function | Behaviour | |---|---| | strings.TrimRight(s, cutset) | Removes all trailing runes that appear anywhere in cutset, one by one, until a rune not in the set is found. | | strings.TrimSuffix(s, suffix) | Removes the exact trailing suffix substring once. | | strings.TrimLeft(s, cutset) | Same as TrimRight but from the left. | | strings.TrimPrefix(s, prefix) | Same as TrimSuffix but from the left. | | strings.Trim(s, cutset) | Applies both TrimLeft and TrimRight. |

strings.TrimRight("123oxo", "xo")   // "123"   (strips o, x, o individually)
strings.TrimSuffix("123oxo", "xo")  // "123o"  (strips the suffix "xo" once)

Rule: When you want to remove a known fixed string from the start/end, use TrimPrefix / TrimSuffix. When you want to strip a set of characters, use TrimLeft / TrimRight / Trim.

4. String concatenation

Avoid += in loops

Strings are immutable. Every += allocates a new string and copies all previous bytes. This is O(n^2) for n concatenations.

Wrong:

func concat(values []string) string {
    s := ""
    for _, v := range values {
        s += v // new allocation every iteration
    }
    return s
}

Use strings.Builder in loops

func concat(values []string) string {
    var sb strings.Builder
    for _, v := range values {
        sb.WriteString(v)
    }
    return sb.String()
}

Pre-grow the builder when total size is known or estimable

Computing the total byte length first and calling Grow avoids internal slice re-allocations. Benchmarks show ~78% speedup over a non-pre-allocated Builder.

func concat(values []string) string {
    total := 0
    for _, v := range values {
        total += len(v)
    }
    var sb strings.Builder
    sb.Grow(total)
    for _, v := range values {
        sb.WriteString(v)
    }
    return sb.String()
}

When += is fine

For a small, fixed number of concatenations (roughly []byte conversions

Each conversion between string and []byte allocates and copies because strings are immutable.

Rule: If input arrives as []byte (from io.Reader, io.ReadAll, etc.) and the result is also []byte, keep the entire pipeline in []byte. The bytes package mirrors almost every function in strings:

| strings function | bytes equivalent | |---|---| | strings.TrimSpace | bytes.TrimSpace | | strings.Split | bytes.Split | | strings.Contains | bytes.Contains | | strings.Index | bytes.Index | | strings.Count | bytes.Count |

Wrong — double conversion:

func getBytes(reader io.Reader) ([]byte, error) {
    b, err := io.ReadAll(reader)
    if err != nil {
        return nil, err
    }
    return []byte(strings.TrimSpace(string(b))), nil // 2 extra allocs
}

Correct — stay in []byte:

func getBytes(reader io.Reader) ([]byte, error) {
    b, err := io.ReadAll(reader)
    if err != nil {
        return nil, err
    }
    return bytes.TrimSpace(b), nil // zero extra allocs
}

Guideline: Before reaching for the strings package, check whether the whole workflow can use bytes instead.

6. Substrings and memory leaks

Substring expressions like s[a:b] produce a string that shares the same backing array as the original. If the original string is large (e.g., a log line of thousands of bytes) and you store only a small substring, the entire original backing array stays in memory and cannot be garbage-collected.

Leaky:

uuid := log[:36]  // uuid's backing array still holds the full log string
s.store(uuid)

Fix: copy the substring

Go >= 1.18 — use strings.Clone:

uuid := strings.Clone(log[:36]) // fresh 36-byte allocation

Pre-1.18 — manual deep copy:

uuid := string([]byte(log[:36])) // forces a new backing array

Note: some linters flag string([]byte(...)) as a "redundant conversion" — this warning is incorrect in this context because the round-trip deliberately breaks backing-array sharing.

Substring indexing caveat

s[a:b] operates on byte indices, not rune indices. For multi-byte runes, convert to []rune first:

s := "Hêllo, World!"
sub := string([]rune(s)[:5]) // "Hêllo" — rune-safe

Quick-reference checklist

• [ ] len(s) used only when byte count is intended; utf8.RuneCountInString for rune count.
• [ ] String range loops use the value variable (r), not s[i], for rune access.
• [ ] TrimRight/TrimLeft not confused with TrimSuffix/TrimPrefix.
• [ ] No += concatenation inside loops; strings.Builder with Grow preferred.
• [ ] No unnecessary string []byte round-trips; bytes package used when possible.
• [ ] Substrings of large strings copied via strings.Clone (or string([]byte(...))) before long-term storage.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: v0lka
• Source: v0lka/skills
• License: MIT

Install and usage instructions live in the source repository linked above.