PHP Regular Expressions

Regular expression can be defined as pattern matching algorithm generated in a single line. These are impactful in case of validation checking and for template recognitions. The meta characters enables user to handle complex patterns. Thus the support in PHP for regular expression helps to enhance code quality of a PHP programming. Any regular expression is generic pattern or sequence of a set of characters which is used to provide pattern matching functionality against a given subject string. It is also known as RegExp or RegEx. It is also considered as pattern notation based small programming language to be used for text string parsing.

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2 Sets of Regular Expressions in PHP Functions

Given below supports 2 sets of regular expressions:

• POSIX Regular Expressions
• PERL Style Regular Expressions

1. POSIX Regular Expressions

This is defined as set of character where any of its single character needs to match from the input string. These expressions are defined within [].

Example:

• [0-9]: This is designed to filter any decimal string from 0 through 9.
• [a-Z]: This is designed to filter any character from lowercase ‘a’ through uppercase ‘Z’.

In order to make filter to be more specific, a standard syntax, including regex and special characters is developed which are known as Quantifiers. It also provides information regarding frequency i.e. the number of occurrence or instances of bracketed character or a group of characters and quantity.

Table to find out descriptions for different quantifiers:

For the POSIX regex,PHP incorporates various functions to carry out various operation using POSIX-style Regexes.

The functions are described as shown in the below table:

2. PERL Style Regular Expressions

This type of Regex patterns similar to POSIX regex but created with meta characters and identifiers. Syntax for this Regexes are interchangeable with POSIX style.

a. Meta characters: An alphabet character preceded by a backslash representing a specific meaning is known as a Meta character.

There are various meta characters that are supported in PHP scripting, being used as Perl type Regex as discussed below:

b. Modifiers:

Similar to POSIX regex function, PHP also offers some specific functions that are compatible with PERL style regex.

Some of the major functions are discussed as below:

Example of PHP Regular Expressions

The below example demonstrates the application

The code snippet is designed to scan through the input string and split the given input into multiple elements by defining the given Regex as boundary.

Code:

<?php
// Declaring a regex
$regex = "([0-9]+)";
// Defining the input string
$inputstr = "String_a 1 String_b 2 String_c 3";
//Splitting the input string based on matching regex expression
$result = preg_split ($regex, $inputstr);
// Displaying result
echo $result[0];
echo "\n";
echo $result[1];
echo "\n";
echo $result[2];
echo "\n";
?>

Output

The function preg_split() has split the input string into 3 parts as the elements ‘1’, ‘2, and ‘3’ are marked as boundaries.

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