Regular Expressions in PHP

PHP Regular Expressions - Full Documentation ↗

What are Regular Expressions?

Regular expressions (regex) are patterns used to match, search, and manipulate character combinations in strings. They’re useful for validation, data extraction, and text processing.

PHP uses PCRE (Perl Compatible Regular Expressions) with functions starting with preg_.

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Basic Syntax

Patterns are enclosed in delimiters (usually /):

Syntax for creating a regular expression:

$pattern = '/your-pattern-here/';     // Basic
$pattern = '/your-pattern-here/i';    // With modifier (case-insensitive)

Essential Metacharacters

Character	Meaning	Example
.	Any character	/c.t/ matches “cat”, “cut”
*	Zero or more	/ca*t/ matches “ct”, “cat”, “caat”
+	One or more	/ca+t/ matches “cat”, “caat”
?	Zero or one	/colou?r/ matches “color”, “colour”
^	Start of string	/^Hello/
$	End of string	/world$/
\d	Any digit	/\d+/ matches numbers
\w	Word character	/\w+/ matches letters/digits
\s	Whitespace	/\s+/ matches spaces

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Core Functions

Find First Match - preg_match()

Searches for the first occurrence of a pattern in a string and stops after finding it.

Syntax: preg_match(string $pattern, string $subject, array &$matches = null, int $flags = 0, int $offset = 0)

Parameters:

• $pattern - The regex pattern to search for
• $subject - The input string to search in
• $matches - Optional array to store captured matches
• $flags - Optional flags (PREG_OFFSET_CAPTURE, etc.)
• $offset - Optional starting position

Returns:

• 1 if pattern matches
• 0 if no match found
• false on error

Example of string matching:

$text = "The year is 2023";
$result = preg_match('/\d{4}/', $text, $matches);
// $result = 1, $matches[0] = "2023"

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Find All Matches - preg_match_all()

Searches for all occurrences of a pattern in a string.

Syntax: preg_match_all(string $pattern, string $subject, array &$matches = null, int $flags = PREG_PATTERN_ORDER, int $offset = 0)

Parameters:

• $pattern - The regex pattern to search for
• $subject - The input string to search in
• $matches - Array to store all captured matches
• $flags - How to order results (PREG_PATTERN_ORDER, PREG_SET_ORDER, etc.)
• $offset - Optional starting position

Returns:

• Number of full pattern matches (0 or more)
• false on error

$text = "Email: user@site.com or admin@site.com";
$count = preg_match_all('/[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}/', $text, $matches);
// $count = 2, $matches[0] = ["user@site.com", "admin@site.com"]

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Replace Patterns - preg_replace()

Replaces all occurrences of a pattern in a string.

Syntax: preg_replace(string|array $pattern, string|array $replacement, string|array $subject, int $limit = -1, int &$count = null)

Parameters:

• $pattern - The regex pattern(s) to search for
• $replacement - The replacement string(s)
• $subject - The input string(s) to search and replace in
• $limit - Maximum replacements per pattern (default: -1 = no limit)
• $count - Variable to store number of replacements made

Returns:

• String/array with replacements made
• null on error

$phone = "123-456-7890";
$formatted = preg_replace('/(\d{3})-(\d{3})-(\d{4})/', '($1) $2-$3', $phone);
// Returns: "(123) 456-7890"

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Split String by Pattern - preg_split()

Splits a string by a pattern.

Syntax: preg_split(string $pattern, string $subject, int $limit = -1, int $flags = 0)

Parameters:

• $pattern - The regex pattern to split by
• $subject - The input string to split
• $limit - Maximum number of pieces (default: -1 = no limit)
• $flags - Optional flags (PREG_SPLIT_NO_EMPTY, etc.)

Returns:

• Array of string pieces
• false on error

$text = "apple,banana;orange:grape";
$fruits = preg_split('/[,;:]/', $text);
// Returns: ["apple", "banana", "orange", "grape"]

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Common Patterns

Common patterns:

// Email validation
$email = '/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/';

// Phone number (US)
$phone = '/^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/';

// URL
$url = '/^https?:\/\/[\w\.-]+\.[a-zA-Z]{2,}(\/.*)?$/';

// Date (YYYY-MM-DD)
$date = '/^\d{4}-\d{2}-\d{2}$/';

// Password (8+ chars, mixed case, number, special)
$password = '/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/';

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Modifiers

Modifiers:

'/pattern/i'    // Case-insensitive
'/pattern/m'    // Multiline (^ and $ match line starts/ends)
'/pattern/s'    // Dot matches newlines
'/pattern/x'    // Ignore whitespace in pattern

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Capturing Groups

Capturing groups:

$date = "2023-12-25";
$pattern = '/(\d{4})-(\d{2})-(\d{2})/';

if (preg_match($pattern, $date, $matches)) {
    echo $matches[1]; // Year: 2023
    echo $matches[2]; // Month: 12
    echo $matches[3]; // Day: 25
}

// Named groups
$pattern = '/(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})/';
if (preg_match($pattern, $date, $matches)) {
    echo $matches['year']; // 2023
}

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Best Practices

• Escape special characters: Use preg_quote() for user input,
• Be specific: Use [^"]* instead of .*? when possible,
• Validate patterns: Check preg_last_error() for debugging,
• Use non-capturing groups: (?:...) when you don’t need the match.

Best practices:

// Safe pattern creation
$userInput = "$29.99";
$escaped = preg_quote($userInput, '/');
$pattern = '/' . $escaped . '/';