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 <= 5) or one-off formatting, +=, fmt.Sprintf, or + are acceptable and more readable.

Safety note

strings.Builder is not safe for concurrent use.

5. Avoid useless string <-> []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.