Character classes

PHP course · Advanced · Lesson 44/92

Learn to narrow patterns with character classes like \d, \w, \s and [a-z].

Character classes: [a-z] and \d

To match any character from a set, use a character class written inside square brackets: [aeiou] matches any vowel.

A hyphen gives a range: [a-z] lowercase letters, [0-9] digits. A ^ at the start means not: [^0-9] matches anything that is not a digit.

There are shortcuts too: \d = [0-9], \w = [a-zA-Z0-9_], \s whitespace. The uppercase forms (\D, \W, \S) are the negated versions.

Ranges and negation

Inside a character class, a hyphen (-) specifies a range. [0-9] is one digit, [A-Fa-f0-9] is one hexadecimal digit.

If the first character is ^, the class is negated: [^0-9] matches any single character that is not a digit. A ^ not at the start is treated as a literal.

To match a literal hyphen, put it at the start or end of the class: [-a] or [a-].

Exercise Only letters and digits?

Write isAlphanumeric(string $str): bool. Using the pattern /^[a-zA-Z0-9]+$/, return true if the string contains only letters and digits, false otherwise. Compare the preg_match result with === 1. The three echo calls below are already written; they print results for "PHP8", "PHP 8" and "PHP-8".

Show example solution

bool { return preg_match("/^[a-zA-Z0-9]+$/", $str) === 1; }
Example solution:
<?php
// isAlphanumeric: only letters and digits?
// Use a [a-zA-Z0-9] pattern with preg_match

// Define the isAlphanumeric function
function isAlphanumeric(string $str): bool {
    return preg_match("/^[a-zA-Z0-9]+$/", $str) === 1;
}

echo isAlphanumeric("PHP8") ? "yes\n" : "no\n";
echo isAlphanumeric("PHP 8") ? "yes\n" : "no\n";
echo isAlphanumeric("PHP-8") ? "yes\n" : "no\n";

Test yourself

Understand the shortcuts

\d

Digit
\w

Word character
\s

Whitespace
\D

Non-digit
[a-z]

Lowercase range
What does [a-z] match?
- Only the letters 'a' and 'z'
- Both uppercase and lowercase letters
- Digits
- One lowercase letter from a to z
Show answer

One lowercase letter from a to z — Correct. [a-z] matches one lowercase letter between a and z.

Add a digit range

$pattern = "/[____]+/";
preg_match($pattern, "Tel: 0555", $matches);

Show answer

$pattern = "/[0-9]+/";
preg_match($pattern, "Tel: 0555", $matches);

Character classes