PHP Regular Expressions

A detailed explanation of PHP Regular Expressions with practical examples. Learn all about delimiters, metacharacters, modifiers & PHP Regex Functions to handle regular expressions:

In this tutorial, you will learn what a regular expression is, delimiters, metacharacters, uses of the backslash character, modifiers, character classes, PHP regex functions, and frequently asked questions (FAQs).

Please note that we have used PHP version 7 in all examples.

Let’s begin!

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Getting Started with PHP Regular Expressions

What is a Regular Expression

A regular expression is a sequence of characters that helps to search text and carry out operations related to text replacement. It is commonly known as regex or RegExp. Regular expressions are case-sensitive by default. Furthermore, PHP provides built-in support for regular expressions.

Regular expressions are useful when validating user inputs like email, password, phone number, URL, IP address, etc. Further, they can use it for highlighting keywords in search results.

There are two types of syntax for writing regular expressions in PHP. They are:

1. POSIX (Portable Operating System Interface) standard
2. PCRE (Perl Compatible Regular Expressions) or Perl syntax

Out of the above two syntaxes, Perl syntax is the most widely used syntax.

You may use the PHP regex tester website (Click the URL) to test and debug your regex.

Delimiters

Delimiters are used to enclose the pattern in PCRE functions. The pattern is surrounded by two delimiters.

Non-alphanumeric, non-backslash, and non-whitespace characters are used as delimiters. The following list shows some of the characters that can be used as delimiters:

• / (forward slash)
• # (hash sign)
• ~ (tildes)
• + (plus sign)
• % (percentage sign)

Examples:

• /foo/
• /foo bar/
• #[a-z]#
• #http://#
• %[a-z]%

Metacharacters

In simple terms, a metacharacter is a character that has a special meaning. Metacharacters are commonly used in regular expressions.

The following table shows the standard regular expression metacharacters:

Examples:

/[abc]/ matches abc

/1-3/ matches 1, 2, 3

/ex*/ matches expression, exercise, etc.

Uses of Backslash

The backslash (\) character has many uses. We have discussed four usages below:

Usage 1: As an escape character

If a backslash character is followed by a non-alphanumeric character, it removes any special meaning that it would have.

Example:

\* matches a * character in the pattern.

Usage 2: To encode non-printing characters in patterns in a visible way.

Another use of the backslash (\) is to encode non-printing characters in patterns in a visible way, as shown in the table below:

Usage 3: To specify the generic character type, as shown in the table below:

Usage 4: Simple assertions as shown below:

Modifiers

Modifiers specify how a search needs to be performed.

The following table shows Pattern Modifiers:

Brackets

Brackets state a range of characters as shown below:

Apart from the above ranges, you may use some other ranges as well.

Examples:

[1-4] – Matches any decimal digit between one and four.

[b-m] – Matches any character from b (lowercase) to m (lowercase).

PHP Regex Functions

PHP has several built-in functions to handle regular expressions, as shown in the table below:

Given below are some examples of regular expression functions.

Example 1: preg_filter() function

<?php
 
$txt = 'John Doe';
$pattern = '/john/i';
 
echo preg_filter($pattern, 'Jane', $txt);
 
?>

Output:

Jane Doe

Example 2: preg_filter() function

<?php
 
$txt = 'John Doe';
$pattern = '/jack/i';
 
echo preg_filter($pattern, 'Jane', $txt);
 
?>

Output:

(no output)

Note: The PHP preg_filter() function and the PHP preg_replace() function work similarly, but the preg_filter() function only returns an output if only finds a match.

Example 3: preg_replace() function

<?php
 
$txt = 'John Doe';
$pattern = '/john/i';
 
echo preg_replace($pattern, 'Jane', $txt);
 
?>

Output:

Jane Doe

Example 4: preg_replace() function

<?php
 
$txt = 'John Doe';
$pattern = '/jack/i';
 
echo preg_replace($pattern, 'Jane', $txt);
 
?>

Output:

John Doe

Example 5: preg_grep() function

<?php
 
$animals = array("Lion","tiger","leopard","wolf","cheetah");
$result = preg_grep("/^l/i", $animals);
 
print_r($result);
 
?>

Output:

Array ( [0] => Lion [2] => leopard )

Example 6: preg_split() function

<?php
 
$txt = 'Software';
$chars = preg_split('//', $txt, -1, PREG_SPLIT_NO_EMPTY);
 
print_r($chars);
 
?>

Output:

Array ( [0] => S [1] => o [2] => f [3] => t [4] => w [5] => a [6] => r [7] => e )

Example 7: preg_match() function

<?php
 
$user_email = "name@user.com";
 
if (preg_match("/^[0-9a-zA-Z.-_]+@[0-9a-zA-Z-]+\.[a-zA-Z.]{2,9}$/", $user_email)) {
         echo "$user_email is valid.";
}else{
         echo "$user_email is invalid.";
}
 
?>

Output:

name@user.com is valid.

Frequently Asked Questions

Conclusion

A regular expression is a sequence of characters that helps to match patterns. They can be used for searching text and carrying out operations related to text replacement.

Some characters in regex have special meanings. Furthermore, PHP provides built-in support for regular expressions.

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