How can PHP developers ensure that only specific characters are allowed in a user input string?

To ensure that only specific characters are allowed in a user input string, PHP developers can use regular expressions to validate the input against a predefined pattern. By defining a regular expression pattern that includes only the allowed characters, developers can easily check if the user input string contains any disallowed characters. 

$userInput = "abc123"; // User input string to validate
$allowedPattern = "/^[a-zA-Z0-9]+$/"; // Regular expression pattern allowing only letters and numbers

if (preg_match($allowedPattern, $userInput)) {
    echo "Input is valid";
} else {
    echo "Input contains disallowed characters";
}

Keywords

PHP validation regular expressions preg_match input sanitization

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