PHP preg_match(): Regular Expressions (Regex)

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 address, domain names, telephone numbers, 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 functions which we will learn in this PHP Regular Expressions tutorial. Let’s look at the commonly used regular expression functions in PHP.

• preg_match() in PHP – this function is used to perform pattern matching in PHP 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 PHP preg_match(), PHP preg_split() or PHP preg_replace().

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

HERE,

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

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

Preg_match() in PHP

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

The code below shows the implementation for preg_match() tester 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’s 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.

The diagram below summarizes the above points

PHP Preg_split()

Let’s now look at another example that uses the preg_split() in PHP 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’s now look at the preg_replace() in 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 colour.

<?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, browser to the URL http://localhost/phptuts/preg_replace.php

Regular Expression Metacharacters

The above examples used very basic patterns; metacharacters simply allow us to perform more complex pattern matches such as test the validity of an email address. Let’s now look at the commonly used metacharacters.

Metacharacter	Description	Example
.	Matches any single character except a new line	/./ matches anything that has a single character
^	Matches the beginning of or string / excludes characters	/^PH/ matches any string that starts with PH
$	Matches pattern at the end of the string	/com$/ matches guru99.com,yahoo.com Etc.
*	Matches any zero (0) or more characters	/com*/ matches computer, communication etc.
+	Requires preceding character(s) appear at least once	/yah+oo/ matches yahoo
\	Used to escape meta characters	/yahoo+\.com/ treats the dot as a literal value
[…]	Character class	/[abc]/ matches abc
a-z	Matches lower case letters	/a-z/ matches cool, happy etc.
A-Z	Matches upper case letters	/A-Z/ matches WHAT, HOW, WHY etc.
0-9	Matches any number between 0 and 9	/0-4/ matches 0,1,2,3,4

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

Let’s 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 pattern “[/^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$/]”

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 example breakdown, metacharacters are very powerful when it comes to matching patterns.

Summary

• A Regular Expression or Regex in PHP is a pattern match algorithm
• Regular expressions are very useful when performing validation checks, creating HTML template systems that recognize tags etc.
• PHP has built in functions namely PHP preg_match(), PHP preg_split() and PHP preg_replace() that support regular expressions.
• Metacharacters allow us to create complex patterns