Usernames may only contain letters and numbers

Some possible ways to answer the question are:

1. If we are designing a system that allows users to create usernames, we should enforce the rule that usernames may only contain letters (a-z and A-Z) and numbers (0-9). To do this, we can use regular expressions to validate the input and reject any username that contains non-alphanumeric characters or symbols. Here is an example code in Python:

python
import re
def is_valid_username(username):
    pattern = r'^[a-zA-Z0-9]+$'
    return bool(re.match(pattern, username))
print(is_valid_username("john123"))  # True
print(is_valid_username("mary's"))   # False
print(is_valid_username("bob#007"))  # False

This function checks if a given `username` matches the pattern `^[a-zA-Z0-9]+$`, which means it should start (`^`) and end (`$`) with one or more alphanumeric characters (`[a-zA-Z0-9]`). If the match succeeds, the function returns `True`, otherwise it returns `False`.

2. If we are processing a list or database of usernames that may contain invalid characters, we may want to sanitize them by removing or replacing the offending characters. For example, we may want to replace spaces, underscores, or dots with hyphens, or remove any punctuation or whitespace altogether. Here is an example code in JavaScript:

javascript
function sanitize_username(username) {
  return username.trim().replace(/\W+/g, "-"); // replace non-word chars with hyphens
}
let usernames = ["john123", "mary's", "bob#007", "jane smith", "john_doe", "jim.baker"];
let sanitized = usernames.map(sanitize_username);
console.log(sanitized); // ["john123", "mary-s", "bob-007", "jane-smith", "john-doe", "jim-baker"]

This function removes any leading or trailing whitespace from the `username` and replaces any sequence of one or more non-word characters (`/\W+/g`) with a single hyphen (`-`). The `map()` method applies this function to each element of the `usernames` array and returns a new array of sanitized usernames. The output shows the result of this operation.

3. If we are displaying usernames on a website or application, we may want to escape or encode them to prevent cross-site scripting (XSS) attacks or HTML injection. For example, we may want to replace certain characters with their HTML entity codes or use a library that automatically sanitizes user input. Here is an example code in PHP:

php
function escape_username($username) {
  return htmlspecialchars($username, ENT_QUOTES | ENT_HTML5, 'UTF-8');
}
$usernames = ["john123", "mary's", "bob#007"];
$escaped = array_map('escape_username', $usernames);
foreach ($escaped as $username) {
  echo "
Welcome, $username!
"; // safe output
}

This function uses the `htmlspecialchars()` function to replace any special characters that have a corresponding HTML entity code (e.g., `&` becomes `&`, `<` becomes `<`, `>` becomes `>`, and `"` becomes `"`). It also specifies the `ENT_QUOTES` and `ENT_HTML5` flags to handle both single and double quotes and the latest HTML5 standards. The `array_map()` function applies this function to each element of the `$usernames` array and returns a new array of escaped usernames. The `foreach` loop outputs a `

` tag that includes the escaped `username` variable, which is now safe to render as HTML. The output will display the usernames without any unintended HTML effects.