PHP Regular Expressions


What is a Regular Expression?

A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.

A regular expression can be a single character, or a more complicated pattern.

Regular expressions can be used to perform all types of text search and text replace operations.


Syntax

In PHP, regular expressions are strings composed of delimiters, a pattern and optional modifiers.

$exp = “/w3schools/i”;

In the example above, / is the delimiterw3schools is the pattern that is being searched for, and i is a modifier that makes the search case-insensitive.

The delimiter can be any character that is not a letter, number, backslash or space. The most common delimiter is the forward slash (/), but when your pattern contains forward slashes it is convenient to choose other delimiters such as # or ~.


Regular Expression Functions

PHP provides a variety of functions that allow you to use regular expressions. The preg_match()preg_match_all() and preg_replace() functions are some of the most commonly used ones:

Function Description
preg_match() Returns 1 if the pattern was found in the string and 0 if not
preg_match_all() Returns the number of times the pattern was found in the string, which may also be 0
preg_replace() Returns a new string where matched patterns have been replaced with another string

Using preg_match()

The preg_match() function will tell you whether a string contains matches of a pattern.

Example

Use a regular expression to do a case-insensitive search for “w3schools” in a string:

<?php
$str = “Visit W3Schools”;
$pattern = “/w3schools/i”;
echo preg_match($pattern, $str); // Outputs 1
?>

Using preg_match_all()

The preg_match_all() function will tell you how many matches were found for a pattern in a string.

Example

Use a regular expression to do a case-insensitive count of the number of occurrences of “ain” in a string:

<?php
$str = “The rain in SPAIN falls mainly on the plains.”;
$pattern = “/ain/i”;
echo preg_match_all($pattern, $str); // Outputs 4
?>

Using preg_replace()

The preg_replace() function will replace all of the matches of the pattern in a string with another string.

Example

Use a case-insensitive regular expression to replace Microsoft with W3Schools in a string:

<?php
$str = “Visit Microsoft!”;
$pattern = “/microsoft/i”;
echo preg_replace($pattern, “W3Schools”, $str); // Outputs “Visit W3Schools!”
?>

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