PHP preg_match(): Regular Expressions (Regex)

What is Regular expression in PHP?

PHP regular expressions, also known as regex, are a powerful pattern matching tool that can be performed in a single expression. Regular expressions use special characters such as +, -, and ^ to create complex expressions. They can help you accomplish tasks such as validating email addresses, IP addresses, and more.

Why use regular expressions

• PHP regular expressions simplify identifying patterns in string data by calling a single function. This saves us coding time.
• When validating user input such as email addresses, domain names, telephone numbers, and IP addresses
• Highlighting keywords in search results
• When creating a custom HTML template. Regex in PHP can be used to identify the template tags and replace them with actual data.

Built-in Regular expression Functions in PHP

PHP has built-in functions that allow us to work with regular expressions, which we will learn in this tutorial. Let us look at the commonly used regular expression functions in PHP.

• preg_match() in PHP – this function is used to perform pattern matching on a string. It returns true if a match is found and false if a match is not found.
• preg_split() in PHP – this function is used to perform a pattern match on a string and then split the results into a numeric array.
• preg_replace() in PHP – this function is used to perform a pattern match on a string and then replace the match with the specified text.

Below is the syntax for a regular expression function such as preg_match(), preg_split(), or preg_replace().

<?php
function_name('/pattern/',subject);
?>

HERE,

• “function_name(…)” is either preg_match(), preg_split(), or preg_replace().
• “/…/” the forward slashes denote the beginning and end of our regular expression
• “‘/pattern/'” is the pattern that we need to match
• “subject” is the text string to be matched against

Let us now look at practical examples that implement the above PHP regex functions.

Preg_match() in PHP

The first example uses the preg_match() function to perform a simple pattern match for the word guru in a given URL.

The code below shows the implementation of the preg_match() function for the above example.

<?php
$my_url = "www.guru99.com";
if (preg_match("/guru/", $my_url))
{
echo "the url $my_url contains guru";
}
else
{
echo "the url $my_url does not contain guru";
}
?>

Browse to the URL http://localhost/phptuts/preg_match_simple.php

Let us examine the part of the code responsible for our output, preg_match(‘/guru/’, $my_url). HERE,

• “preg_match(…)” is the PHP regex function
• “‘/guru/'” is the regular expression pattern to be matched
• “$my_url” is the variable containing the text to be matched against.

PHP Preg_split()

Let us now look at another example that uses the preg_split() function.

We will take a string phrase and explode it into an array; the pattern to be matched is a single space.

The text string to be used in this example is “I Love Regular Expressions”.

The code below illustrates the implementation of the above example.

<?php

$my_text = "I Love Regular Expressions";

$my_array = preg_split("/ /", $my_text);

print_r($my_array );

?>

Browse to the URL http://localhost/phptuts/preg_split.php

PHP Preg_replace()

Let us now look at the preg_replace() PHP function that performs a pattern match and then replaces the pattern with something else.

The code below searches for the word guru in a string.

It replaces the word guru with the word guru surrounded by CSS code that highlights the background color.

<?php

$text = "We at Guru99 strive to make quality education affordable to the masses. Guru99.com";

$text = preg_replace("/Guru/", '<span style="background:yellow">Guru</span>', $text);

echo $text;

?>

Assuming you have saved the file preg_replace.php, browse to the URL http://localhost/phptuts/preg_replace.php

Regular Expression Metacharacters

The above examples used very basic patterns. Metacharacters allow us to perform more complex pattern matches, such as testing the validity of an email address. Let us now look at the commonly used metacharacters.

The above list only gives the most commonly used metacharacters in regular expressions.

Let us now look at a fairly complex example that checks the validity of an email address.

<?php
$my_email = "name@company.com";
if (preg_match("/^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$/", $my_email)) {
echo "$my_email is a valid email address";
}
else
{
echo "$my_email is NOT a valid email address";
}
?>

Explaining the Email Validation Pattern

HERE,

• “/…/” starts and ends the regular expression
• “^[a-zA-Z0-9._-]” matches any lower or upper case letters, numbers between 0 and 9, and dots, underscores, or dashes.
• “+@[a-zA-Z0-9-]” matches the @ symbol followed by lower or upper case letters, numbers between 0 and 9, or dashes.
• “+\.[a-zA-Z.]{2,5}$/” escapes the dot using the backslash, then matches any lower or upper case letters with a character length between 2 and 5 at the end of the string.

Browse to the URL http://localhost/phptuts/preg_match.php

As you can see from the above breakdown, metacharacters are very powerful when it comes to matching patterns.

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