Why Does Go's `bufio.Reader.ReadString` Preserve Leading Delimiters, and How Can I Avoid This Issue?

Reader.ReadString May Preserve Leading Delimiters

In Go, bufio.Reader.ReadString behavior can differ when parsing input. Consider the following scenario:

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    reader := bufio.NewReader(os.Stdin)
    fmt.Print("Who are you? Enter your name: ")
    text, _ := reader.ReadString('\n')
    if text == "Alice" || text == "Bob" {
        fmt.Printf("Hello, ", text)
    } else {
        fmt.Printf("You're not allowed in here!")
    }
}

Here, ReadString is used to read user input and expects names terminated by a newline character. However, when the entered name matches "Alice" or "Bob," the program erroneously denies access.

Cause and Solution

This issue arises because ReadString preserves leading occurrences of the delimiter (in this case, 'n'). When a name like "Alice" is entered, the string stored in text includes both "Alice" and a trailing newline. This extra newline character prevents the string comparison from matching the expected value.

To resolve this issue, consider the following approaches:

1. Trim Leading Whitespace: Before comparing text with "Alice" or "Bob," use strings.TrimSpace(text) to remove any leading whitespace, including the newline character.
2. Use ReadLine Instead: Alternatively, switch to reader.ReadLine() instead of ReadString. This function returns the input without including any leading or trailing delimiters. Note that reader.ReadLine() will return bytes; hence, cast it to a string using string(bytes).

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