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4.3 PHP RegEx

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 delimiter, w3schools 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 1: PHP RegEx - Using preg_match()

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 2: PHP RegEx - Using preg_match_all()

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 3: PHP RegEx - Using preg_replace()

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!"
?>

Regular Expression Modifiers

Modifiers can change how a search is performed.

Modifier	Description
i	Performs a case-insensitive search
m	Performs a multiline search (patterns that search for the beginning or end of a string will match the beginning or end of each line)
u	Enables correct matching of UTF-8 encoded patterns

Regular Expression Patterns

Brackets are used to find a range of characters:

Expression	Description
[abc]	Find one character from the options between the brackets
[^abc]	Find any character NOT between the brackets
[0-9]	Find one character from the range 0 to 9

Metacharacters

Metacharacters are characters with a special meaning:

Metacharacter	Description
\|	Find a match for any one of the patterns separated by \| as in: cat\|dog\|fish
.	Find just one instance of any character
^	Finds a match as the beginning of a string as in: ^Hello
$	Finds a match at the end of the string as in: World$
\d	Find a digit
\s	Find a whitespace character
\b	Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b
\uxxxx	Find the Unicode character specified by the hexadecimal number xxxx

Quantifiers

Quantifiers define quantities:

Quantifier	Description
n+	Matches any string that contains at least one n
n*	Matches any string that contains zero or more occurrences of n
n?	Matches any string that contains zero or one occurrences of n
n{x}	Matches any string that contains a sequence of X n's
n{x,y}	Matches any string that contains a sequence of X to Y n's
n{x,}	Matches any string that contains a sequence of at least X n's

Note: If your expression needs to search for one of the special characters you can use a backslash ( \ ) to escape them. For example, to search for one or more question marks you can use the following expression: $pattern = '/\?+/';

Grouping

You can use parentheses ( ) to apply quantifiers to entire patterns. They also can be used to select parts of the pattern to be used as a match.

Example 4: PHP RegEx - Grouping

Use grouping to search for the word "banana" by looking for ba followed by two instances of na:

<?php
$str = "Apples and bananas.";
$pattern = "/ba(na){2}/i";
echo preg_match($pattern, $str); // Outputs 1
?>

Complete RegExp Reference

For a complete reference, go to W3Schools.com Complete PHP Regular Expression Reference.
The reference contains descriptions and examples of all Regular Expression functions.

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Module 4. Arrays, Superglobals and Regex

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PHP Arrays Array Items PHP Arrays - Get The Length of an Array with The count() Function PHP Arrays - Change Value Indexed Arrays - Loop Through an Indexed Array PHP Arrays: Index Number part 1 PHP Arrays: Index Number part 2 PHP Arrays - Associative Arrays part 1 PHP Arrays - Associative Arrays part 2 PHP Arrays - Access Associative Arrays PHP Associative Arrays: Change Key's Value Associative Arrays - Loop Through an Associative Array part 1 Associative Arrays - Loop Through an Associative Array part 2 PHP Arrays - Two-dimensional Arrays PHP Arrays - Two-dimensional Arrays with for loop PHP Arrays - Sorting Arrays part 1 PHP Arrays - Sorting Arrays part 2 PHP Arrays - Sort Array in Descending Order rsort() part 1 PHP Arrays - Sort Array in Descending Order rsort() part 2 PHP Arrays - Sort Array (Ascending Order), According to Value asort() PHP Arrays - Sort Array (Ascending Order), According to Key ksort() PHP Arrays - Sort Array (Descending Order), According to Value arsort() PHP Arrays - Sort Array (Descending Order), According to Key krsort() PHP $GLOBALS PHP $_SERVER PHP $_REQUEST - Form Submission PHP $_POST - Form Submission Test Get PHP RegEx - Using preg_match() PHP RegEx - Using preg_match_all() PHP RegEx - Using preg_replace() PHP RegEx - Grouping