Regular Expressions

🔍 PHP Regular Expressions: The Pattern Detective

Imagine you’re a detective with a magical magnifying glass. This glass can find ANY pattern in a book—names, phone numbers, secret codes—anything! Regular expressions are your magnifying glass for text.

🎯 What Are Regular Expressions?

Think of regular expressions (regex) like a search filter on steroids.

Normal search: Find the word “cat” Regex search: Find anything that looks like a phone number, email, or date—even if you don’t know the exact text!

// Normal search - finds exact match
strpos("I have a cat", "cat"); // ✓ Found!

// Regex search - finds ANY 3-digit number
preg_match("/\d{3}/", "Call 555-1234"); // ✓ Found 555!

🧱 Regular Expression Basics

The Two Delimiters

Every regex pattern in PHP lives between delimiters—like bookends for your pattern:

"/pattern/"   // Forward slashes (most common)
"#pattern#"   // Hash marks (useful when / is in pattern)
"~pattern~"   // Tildes (another option)

Your First Pattern Match

$text = "I love pizza!";

if (preg_match("/pizza/", $text)) {
    echo "Yum! Found pizza!";
}
// Output: Yum! Found pizza!

What happened?

1. /pizza/ = Look for the word “pizza”
2. preg_match() = PHP’s regex search function
3. Returns 1 (true) if found, 0 (false) if not

🎨 Regular Expression Patterns

📌 Basic Pattern Characters

📌 Quantifiers (How Many?)

graph TD
    A["Quantifiers"] --> B["* = Zero or more"]
    A --> C["+ = One or more"]
    A --> D["? = Zero or one"]
    A --> E["{n} = Exactly n times"]
    A --> F["{n,m} = Between n and m"]

Examples:

// * = Zero or more
preg_match("/ab*c/", "ac");     // ✓ (zero b's)
preg_match("/ab*c/", "abbbc");  // ✓ (three b's)

// + = One or more
preg_match("/ab+c/", "ac");     // ✗ (needs at least 1 b)
preg_match("/ab+c/", "abc");    // ✓ (one b)

// {n,m} = Between n and m
preg_match("/\d{3,5}/", "1234"); // ✓ (4 digits)

📌 Character Classes (Pick One From Group)

Character classes let you say “match any ONE of these characters”:

// [abc] = Match a, b, OR c
preg_match("/[aeiou]/", "hello"); // ✓ Finds 'e'

// [0-9] = Any digit (same as \d)
preg_match("/[0-9]+/", "Room 42"); // ✓ Finds 42

// [A-Za-z] = Any letter
preg_match("/[A-Za-z]+/", "PHP8"); // ✓ Finds PHP

// [^abc] = NOT a, b, or c
preg_match("/[^0-9]/", "R2D2"); // ✓ Finds R

📌 Groups and Alternation

Groups () = Capture parts of a match Alternation | = This OR that

// Alternation: cat OR dog
preg_match("/cat|dog/", "I love dogs"); // ✓ Finds dog

// Groups: Capture the match
preg_match("/(\d{3})-(\d{4})/", "555-1234", $m);
echo $m[1]; // 555
echo $m[2]; // 1234

📌 Special Escape Characters

// To match actual special characters, escape them
preg_match("/\./", "file.txt");     // Match literal dot
preg_match("/\$/", "$100");         // Match dollar sign
preg_match("/\?/", "Really?");      // Match question mark

🔧 Regular Expression Functions

PHP gives you powerful tools to work with regex:

graph TD
    A["PHP Regex Functions"] --> B["preg_match#40;#41;<br/>Find first match"]
    A --> C["preg_match_all#40;#41;<br/>Find ALL matches"]
    A --> D["preg_replace#40;#41;<br/>Search & replace"]
    A --> E["preg_split#40;#41;<br/>Split string by pattern"]

🔍 preg_match() — Find First Match

$email = "contact@example.com";
$pattern = "/[a-z]+@[a-z]+\.[a-z]+/";

if (preg_match($pattern, $email)) {
    echo "Valid email format!";
}

With captures:

$text = "My age is 25 years";
preg_match("/age is (\d+)/", $text, $matches);

echo $matches[0]; // "age is 25" (full match)
echo $matches[1]; // "25" (captured group)

🔍 preg_match_all() — Find ALL Matches

$text = "Prices: $10, $25, $99";
preg_match_all("/\$(\d+)/", $text, $matches);

print_r($matches[1]);
// Array: [10, 25, 99]

🔄 preg_replace() — Search and Replace

// Replace all digits with X
$phone = "555-123-4567";
echo preg_replace("/\d/", "X", $phone);
// Output: XXX-XXX-XXXX

// Use captured groups in replacement
$name = "Smith, John";
echo preg_replace("/(\w+), (\w+)/", "$2 $1", $name);
// Output: John Smith

✂️ preg_split() — Split by Pattern

// Split by any whitespace
$text = "apple   banana\tcherry";
$fruits = preg_split("/\s+/", $text);
// Array: [apple, banana, cherry]

// Split by comma OR semicolon
$data = "a,b;c,d";
$parts = preg_split("/[,;]/", $data);
// Array: [a, b, c, d]

🎯 Pattern Modifiers

Add these after the closing delimiter to change behavior:

// i = Case-insensitive
preg_match("/hello/i", "HELLO"); // ✓ Matches!

// m = Multiline (^ and $ match line starts/ends)
preg_match("/^line/m", "first\nline two"); // ✓

// s = Dot matches newlines too
preg_match("/a.b/s", "a\nb"); // ✓ Matches!

// Combine modifiers
preg_match("/hello world/is", "HELLO\nWORLD"); // ✓

🌟 Real-World Examples

Validate an Email

$pattern = "/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/";

preg_match($pattern, "user@site.com"); // ✓
preg_match($pattern, "bad-email");      // ✗

Extract Hashtags

$tweet = "Learning #PHP and #regex is fun!";
preg_match_all("/#(\w+)/", $tweet, $tags);

print_r($tags[1]); // [PHP, regex]

Clean Phone Number

$phone = "(555) 123-4567";
$clean = preg_replace("/[^0-9]/", "", $phone);
echo $clean; // 5551234567

Find URLs

$text = "Visit https://example.com today!";
preg_match("/https?:\/\/[^\s]+/", $text, $url);
echo $url[0]; // https://example.com

🧠 Quick Memory Tips

. = Any character (like a wildcard card in a game) * = Zero or more (optional, greedy) + = One or more (required, greedy) ? = Optional (zero or one) \d = Digit (0-9) \w = Word character (letters, numbers, underscore) [] = Choose one from the set () = Capture group (save for later) | = OR (this or that)

🚀 You Did It!

You now have the magical magnifying glass of PHP! Regular expressions might look scary at first—like a secret code—but they’re just patterns describing what you want to find.

Start simple. Build up. Practice often.

The more you use regex, the more powerful your text-searching superpowers become! 🦸‍♂️