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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 = “/CodeDixa/i”;

In the example above, / is the delimiterCodeDixa 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:

FunctionDescription
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 “www.php.net” in a string:

<?php
$str = "Visit www.php.net";
$pattern = "/www.php.net/i";
echo preg_match($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, "www.php.net", $str); // Outputs "Visit www.php.net!"
?>

Regular Expression Modifiers

Modifiers can change how a search is performed.

ModifierDescription
iPerforms a case-insensitive search
mPerforms a multiline search (patterns that search for the beginning or end of a string will match the beginning or end of each line)
uEnables correct matching of UTF-8 encoded patterns

Regular Expression Patterns

Brackets are used to find a range of characters:

ExpressionDescription
[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:

MetacharacterDescription
|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
$Finds a match at the end of the string as in: World$
\dFind a digit
\sFind a whitespace character
\bFind a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b
\uxxxxFind the Unicode character specified by the hexadecimal number xxxx

Quantifiers

Quantifiers define quantities:

QuantifierDescription
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

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
?>

Credit: – www.php.net

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