Blog

In this article, we’ll see PHP Regular Expressions.

Welcome to the captivating realm of PHP regular expressions, where the power of patterns can transform your text manipulation skills.

We’ll embark on an exciting journey to unravel the mysteries of PHP regular expressions.

Together, we’ll learn the language of patterns, discover the wonders of pattern matching, and unlock the secrets to becoming a pattern magician.

What are PHP Regular Expressions?

PHP Regular expressions are a powerful tool for examining and modifying text.

PHP Regular expressions themselves, with a general pattern notation almost like a mini programming language, allow you to describe and parse text.

They enable you to search for patterns within a string, extracting matches flexibly and precisely. They are also slower than the more basic string functions.

Basic Of PHP Regular Expressions:

In a regular expression, most characters match only themselves.

For instance, if you search for the php regular expressions “foo” in the string “John plays football,” you get a match because “foo” occurs in that string. Some characters have special meanings in PHP regular expressions.

For instance, a dollar sign ($) is used to match strings that end with the given pattern. Similarly, a caret (^) character at the beginning of php regular expressions indicates that it must match the beginning of the string.

The characters that match themselves are called literals. The characters that have special meanings are called metacharacters.

The dot (.) metacharacter matches any single character except newline (\).

So, the pattern h.t matches hat, hothit, hut, h7t, etc. The vertical pipe (|) metacharacter is used for alternatives in a regular expression.

It behaves much like a logical OR operator and you should use it if you want to construct a pattern that matches more than one set of characters.

For instance, the pattern Utah|Idaho|Nevada matches strings that contain “Utah” or “Idaho” or “Nevada”. Parentheses give us a way to group sequences.

For example, (Nant|b)ucket matches “Nantucket” or “bucket”. Using parentheses to group together characters for alternation is called grouping.

If you want to match a literal metacharacter in a pattern, you have to escape it with a backslash.

To specify a set of acceptable characters in your pattern, you can either build a character class yourself or use a predefined one.

A character class lets you represent a bunch of characters as a single item in php regular expressions. You can build your own character class by enclosing the acceptable characters in square brackets.

A character class matches any one of the characters in the class.

For example a character class [abc] matches a, b or c. To define a range of characters, just put the first and last characters in, separated by hyphen.

For example, to match all alphanumeric characters: [a-zA-Z0-9]. You can also create a negated character class, which matches any character that is not in the class.

To create a negated character class, begin the character class with ^: [^0-9].

The metacharacters +, *, ?, and {} affect the number of times a pattern should be matched.

+ means “Match one or more of the preceding expression”, * means “match zero or more of the preceding expression”, and? means “match zero or one of the preceding expressions”.

Curly braces {} can be used differently.

With a single integer, {n} means “match exactly n occurrences of the preceding expression”, with one integer and a comma, {n,} means “match n or more occurrences of the preceding expression”, and with two comma-separated integers {n,m} means “match the previous character if it occurs at least n times, but no more than m times”.

Now, have a look at the examples:

Perl-Compatible Regular Expressions emulate the Perl syntax for patterns, which means that each pattern must be enclosed in a pair of delimiters.

Usually, the slash (/) character is used. For instance, /pattern/.

The PCRE functions can be divided in several classes: matching, replacing, splitting and filtering.

Matching Patterns:

The preg_match() function performs Perl-style pattern matching on a string. preg_match() takes two basic and three optional parameters.

These parameters are, in order, a regular expression string, a source string, an array variable which stores matches, a flag argument and an offset parameter that can be used to specify the alternate place from which to start the search:

The preg_match() function returns 1 if a match is found and 0 otherwise. Let’s search the string “Hello World!” for the letters “ll”:

<?php
if (preg_match("/ell/", "Hello World!", $matches)) {
echo "Match was found <br />";
echo $matches[0];
}
?>

The letters “ll” exist in “Hello”, so preg_match() returns 1 and the first element of the $matches variable is filled with the string that matched the pattern.

The php regular expressions in the next example is looking for the letters “ell”, but looking for them with following characters:

<?php
if (preg_match("/ll.*/", "The History of Halloween", $matches)) {
echo "Match was found <br />";
echo $matches[0];
}
?>

Now let’s consider more complicated example.

The most popular use of PHP regular expressions is validation. The example below checks if the password is “strong”, i.e. the password must be at least 8 characters and must contain at least one lower case letter, one upper case letter and one digit:

<?php
$password = "Fyfjk34sdfjfsjq7";if (preg_match("/^.*(?=.{8,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).*$/", $password)) {
echo "Your passwords is strong.";
} else {
echo "Your password is weak.";
}
?>

The ^ and $ are looking for something at the start and the end of the string.

The “.*” combination is used at both the start and the end. As mentioned above, the .(dot) metacharacter means any alphanumeric character, and * metacharacter means “zero or more”.

Between are groupings in parentheses. The “?=” combination means “the next text must be like this”. This construct doesn’t capture the text.

In this example, instead of specifying the order that things should appear, it’s saying that it must appear but we’re not worried about the order.

The first grouping is (?=.*{8,}). This checks if there are at least 8 characters in the string.

The next grouping (?=.*[0-9]) means “any alphanumeric character can happen zero or more times, then any digit can happen”.

So this checks if there is at least one number in the string. But since the string isn’t captured, that one digit can appear anywhere in the string.

The next groupings (?=.*[a-z]) and (?=.*[A-Z]) are looking for the lower case and upper case letter accordingly anywhere in the string.

Finally, we will consider php regular expressions that validates an email address:

<?php
$email = firstname.lastname@aaa.bbb.com;
$regexp = "/^[^0-9][A-z0-9_]+([.][A-z0-9_]+)*[@][A-z0-9_]+([.][A-z0-9_]+)*[.][A-z]{2,4}$/";if (preg_match($regexp, $email)) {
echo "Email address is valid.";
} else {
echo "Email address is <u>not</u> valid.";
}
?>

This regular expression checks for the number at the beginning and also checks for multiple periods in the user name and domain name in the email address. Let’s try to investigate this regular expression yourself.

For speed reasons, the preg_match() function matches only the first pattern it finds in a string. This means it is very quick to check whether a pattern exists in a string.

An alternative function, preg_match_all(), matches a pattern against a string as many times as the pattern allows, and returns the number of times it matched.

Replacing Patterns:

In the above examples, we have searched for patterns in a string, leaving the search string untouched.

The preg_replace() function looks for substrings that match a pattern and then replaces them with new text. preg_replace() takes three basic parameters and an additional one.

These parameters are, in order, a regular expression, the text with which to replace a found pattern, the string to modify, and the last optional argument which specifies how many matches will be replaced.

The function returns the changed string if a match was found or an unchanged copy of the original string otherwise.

In the following example we search for the copyright phrase and replace the year with the current.

<?php
echo preg_replace("/([Cc]opyright) 200(3|4|5|6)/", "$1 2007", "Copyright 2005");
?>

In the above example we use back references in the replacement string.

Back references make it possible for you to use part of a matched pattern in the replacement string.

To use this feature, you should use parentheses to wrap any elements of your regular expression that you might want to use.

You can refer to the text matched by subpattern with a dollar sign ($) and the number of the subpattern.

For instance, if you are using subpatterns, $0 is set to the whole match, then $1, $2, and so on are set to the individual matches for each subpattern.

In the following example we will change the date format from “yyyy-mm-dd” to “mm/dd/yyy”:

<?php
echo preg_replace("/(\d+)-(\d+)-(\d+)/", "$2/$3/$1", "2007-01-25");
?>

We also can pass an array of strings as subject to make the substitution on all of them.

To perform multiple substitutions on the same string or array of strings with one call to preg_replace(), we should pass arrays of patterns and replacements. Have a look at the example:

<?php
$search = array ( "/(\w{6}\s\(w{2})\s(\w+)/e",
"/(\d{4})-(\d{2})-(\d{2})\s(\d{2}:\d{2}:\d{2})/");$replace = array ('"$1 ".strtoupper("$2")',
"$3/$2/$1 $4");$string = "Posted by John | 2007-02-15 02:43:41";echo preg_replace($search, $replace, $string);?>

In the above example we use the other interesting functionality – you can say to PHP that the match text should be executed as PHP code once the replacement has taken place.

Since we have appended an “e” to the end of the php regular expressions, PHP will execute the replacement it makes. That is, it will take strtoupper(name) and replace it with the result of the strtoupper() function, which is NAME.

Array Processing

PHP’s preg_split() function enables you to break a string apart basing on something more complicated than a literal sequence of characters.

When it’s necessary to split a string with a dynamic expression rather than a fixed one, this function comes to the rescue.

The basic idea is the same as preg_match_all() except that, instead of returning matched pieces of the subject string, it returns an array of pieces that didn’t match the specified pattern.

The following example uses php regular expressions to split the string by any number of commas or space characters:

<?php
$keywords = preg_split("/[\s,]+/", "php, regular expressions");
print_r( $keywords );
?>

Another useful PHP function is the preg_grep() function which returns those elements of an array that match a given pattern.

This function traverses the input array, testing all elements against the supplied pattern. If a match is found, the matching element is returned as part of the array containing all matches.

The following example searches through an array and all the names starting with letters A-J:

<?php
$names = array('Andrew','John','Peter','Nastin','Bill');
$output = preg_grep('/^[a-m]/i', $names);
print_r( $output );
?>

Congratulations, you’ve become a master of PHP regular expressions!

By understanding the language of patterns, harnessing the magic of pattern matching, and mastering the art of capturing and substituting, you’ve unlocked the secrets of text manipulation.

With your newfound skills, you can tackle complex tasks, validate input, extract data, and transform text with ease. So, embrace the enchantment of PHP regular expressions, and let your patterns weave their magic in your code.

Get ready to dazzle the world with your pattern wizardry!

I hope this article helps to learn PHP regular expressions.