PHP RegEx

Regular Expressions in PHP (preg_* Functions)

Regular expressions (RegEx) are pattern strings used to search, validate, match, and replace complex text formatting in PHP using PCRE (Perl Compatible Regular Expressions).

Essential PHP RegEx Functions

• preg_match($pattern, $subject, $matches): Checks if pattern matches text (returns 1 or 0).
• preg_match_all($pattern, $subject, $matches): Finds all pattern occurrences in string.
• preg_replace($pattern, $replacement, $subject): Replaces matching text pattern.
• preg_split($pattern, $subject): Splits string into array by regex delimiter.

RegEx Pattern Anatomy

flowchart LR
    A["Pattern Delimiter '/'"] --> B["Regex Expression '[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}'"]
    B --> C["Pattern Delimiter '/'"]
    C --> D["Modifiers 'i' (Case-Insensitive)"]

Practical Code Example

<?php
declare(strict_types=1);

// 1. Validating Username Format (Alphanumeric & Underscores, 3-16 chars)
$username = "maksudur_dev";
$usernamePattern = '/^[a-zA-Z0-9_]{3,16}$/';

if (preg_match($usernamePattern, $username)) {
    echo "Username '$username' is VALID.
";
} else {
    echo "Username '$username' is INVALID.
";
}

// 2. Extracting Phone Numbers using preg_match_all
$text = "Contact support at 555-0199 or emergency desk at 555-0822.";
$phonePattern = '/\d{3}-\d{4}/';

preg_match_all($phonePattern, $text, $matches);
echo "Extracted Phone Numbers: " . implode(", ", $matches[0]) . "
";

// 3. Sanitizing Multiple Spaces with preg_replace
$messyString = "PHP   is     a   great    language.";
$cleanString = preg_replace('/\s+/', ' ', $messyString);
echo "Cleaned String: '$cleanString'
";

Best Practices

• Enclose Patterns in Matching Delimiters: Use /pattern/ or #pattern# consistently.
• Use Character Classes & Quantifiers Correctly: Use ^ for string start, $ for string end, and + for 1 or more occurrences.
• Use Dedicated Filter Functions for Common Formats: Use filter_var($email, FILTER_VALIDATE_EMAIL) for email validation instead of writing long fragile custom regexes.

Self-Check Challenge

Write a preg_match() call that validates whether a string $postalCode matches a 5-digit ZIP code pattern (/^\d{5}$/)!