The strings Package

Three functions cover different notions of "contains." Contains checks for a substring, ContainsAny checks for any single character out of a set, and ContainsRune checks for one specific rune.

strings.Contains("seafood", "foo")    // true, "foo" is a substring
strings.ContainsAny("seafood", "xyz") // false, none of x, y, z appear
strings.ContainsRune("seafood", 'f')  // true, 'f' appears

Added in Go 1.18 to replace the older Index plus manual slicing idiom for splitting a string into exactly two parts. The boolean return reports whether the separator was found at all, so a missing separator is distinguishable from an empty side.

// before Cut existed
i := strings.Index("key=value", "=")
key, val := "key=value"[:i], "key=value"[i+1:]

// with Cut
key, val, found := strings.Cut("key=value", "=")
// key = "key", val = "value", found = true

Split returns every piece, including empty ones between adjacent separators. SplitN caps the piece count, leaving the remainder unsplit in the last element. Fields ignores separators entirely and splits on whitespace runs, discarding empty pieces, the right tool for parsing loosely-spaced input.

strings.Split("a,,b", ",")      // [a  b], empty string between the commas
strings.SplitN("a,b,c", ",", 2) // [a b,c], stops after 1 split
strings.Fields("  a   b ")     // [a b], whitespace runs collapsed

A Builder's zero value is ready to use, no constructor needed. It grows one internal buffer as you write to it, instead of allocating a new string on every += concatenation in a loop.

var b strings.Builder
for _, task := range []string{"write the handler", "test it", "ship it"} {
    b.WriteString("- ")
    b.WriteString(task)
    b.WriteByte('\n')
}
fmt.Print(b.String())

// - write the handler
// - test it
// - ship it