PHP | Regular Expressions

Regular expressions commonly known as a regex (regexes) are a sequence of characters describing a special search pattern in the form of text string. They are basically used in programming world algorithms for matching some loosely defined patterns to achieve some relevant tasks. Some times regexes are understood as a mini programming language with a pattern notation which allows the users to parse text strings. The exact sequence of characters are unpredictable beforehand, so the regex helps in fetching the required strings based on a pattern definition. Regular Expression is a compact way of describing a string pattern that matches a particular amount of text. As you know, PHP is an open-source language commonly used for website creation, it provides regular expression functions as an important tool. Like PHP, many other programming languages have their own implementation of regular expressions. This is the same with other applications also, which have their own support of regexes having various syntaxes. Many available modern languages and tools apply regexes on very large files and strings. Let us look into some of the advantages and uses of regular expressions in our applications. Advantages and uses of Regular expressions: In many scenarios, developers face problems whenever data are collected in free text fields as most of the programming deals with data entries. Regular expressions are used almost everywhere in today's application programming.

• Regular expressions help in validation of text strings which are of programmer's interest.
• It offers a powerful tool for analyzing, searching a pattern and modifying the text data.
• It helps in searching specific string pattern and extracting matching results in a flexible manner.
• It helps in parsing text files looking for a defined sequence of characters for further analysis or data manipulation.
• With the help of in-built regexes functions, easy and simple solutions are provided for identifying patterns.
• It effectively saves a lot of development time, which are in search of specific string pattern.
• It helps in important user information validations like email address, phone numbers and IP address.
• It helps in highlighting special keywords in a file based on search result or input.
• It helps in identifying specific template tags and replacing those data with the actual data as per the requirement.
• Regexes are very useful for creation of HTML template system recognizing tags.
• Regexes are mostly used for browser detection, spam filtration, checking password strength and form validations.

We cannot cover everything under this topic, but let us look into some of the major regular expression concepts. The following table shows some regular expressions and the corresponding string which matches the regular expression pattern.

Regular Expression	Matches
geeks	The string "geeks"
^geeks	The string which starts with "geeks"
geeks$	The string which have "geeks" at the end.
^geeks$	The string where "geeks" is alone on a string.
[abc]	a, b, or c
[a-z]	Any lowercase letter
[^A-Z]	Any letter which is NOT a uppercase letter
(gif|png)	Either "gif" or "png"
[a-z]+	One or more lowercase letters
^[a-zA-Z0-9]{1, }$	Any word with at least one number or one letter
([ax])([by])	ab, ay, xb, xy
[^A-Za-z0-9]	Any symbol other than a letter or other than number
([A-Z]{3}|[0-9]{5})	Matches three letters or five numbers

Note: Complex search patterns can be created by applying some basic regular expression rules. Even many arithmetic operators like +, ^, - are used by regular expressions for creating little complex patterns. Operators in Regular Expression: Let us look into some of the operators in PHP regular expressions.

Operator	Description
^	It denotes the start of string.
$	It denotes the end of string.
.	It denotes almost any single character.
()	It denotes a group of expressions.
[]	It finds a range of characters for example [xyz] means x, y or z .
[^]	It finds the items which are not in range for example [^abc] means NOT a, b or c.
– (dash)	It finds for character range within the given item range for example [a-z] means a through z.
| (pipe)	It is the logical OR for example x | y means x OR y.
?	It denotes zero or one of preceding character or item range.
*	It denotes zero or more of preceding character or item range.
+	It denotes one or more of preceding character or item range.
{n}	It denotes exactly n times of preceding character or item range for example n{2}.
{n, }	It denotes atleast n times of preceding character or item range for example n{2, }.
{n, m}	It denotes atleast n but not more than m times for example n{2, 4} means 2 to 4 of n.
\	It denotes the escape character.

Special Character Classes in Regular Expressions: Let us look into some of the special characters used in regular expressions.

Special Character	Meaning
\n	It denotes a new line.
\r	It denotes a carriage return.
\t	It denotes a tab.
\v	It denotes a vertical tab.
\f	It denotes a form feed.
\xxx	It denotes octal character xxx.
\xhh	It denotes hex character hh.

Shorthand Character Sets: Let us look into some shorthand character sets available.

Shorthand	Meaning
\s	Matches space characters like space, newline or tab.
\d	Matches any digit from 0 to 9.
\w	Matches word characters including all lower and upper case letters, digits and underscore.

Predefined functions or Regex library: Let us look into the quick cheat sheet of pre-defined functions for regular expressions in PHP. PHP provides the programmers to many useful functions to work with regular expressions. The below listed in-built functions are case-sensitive.

Function	Definition
preg_match()	This function searches for a specific pattern against some string. It returns true if pattern exists and false otherwise.
preg_match_all()	This function searches for all the occurrences of string pattern against the string. This function is very useful for search and replace.
ereg_replace()	This function searches for specific string pattern and replace the original string with the replacement string, if found.
eregi_replace()	The function behaves like ereg_replace() provided the search for pattern is not case sensitive.
preg_replace()	This function behaves like ereg_replace() function provided the regular expressions can be used in the pattern and replacement strings.
preg_split()	The function behaves like the PHP split() function. It splits the string by regular expressions as its parameters.
preg_grep()	This function searches all elements which matches the regular expression pattern and returns the output array.
preg_quote()	This function takes string and quotes in front of every character which matches the regular expression.
ereg()	This function searches for a string which is specified by a pattern and returns true if found, otherwise returns false.
eregi()	This function behaves like ereg() function provided the search is not case sensitive.

Note:

• By default, regular expressions are case sensitive.
• There is a difference between strings inside single quotes and strings inside double quotes in PHP. The former are treated literally, whereas for the strings inside double-quotes means the content of the variable is printed instead of just printing their names.

Example 1: php

<?php

// Declare a regular expression
$regex = '/^[a-zA-Z ]*$/';

// Declare a string
$nameString = 'Sharukh khan';

// Use preg_match() function to
// search string pattern
if(preg_match($regex, $nameString)) {
    echo("Name string matching with"
        . " regular expression");
}
else {
    echo("Only letters and white space"
        . " allowed in name string"); 
}

?>

Output:

Name string matching with regular expression

Example 2: php

<?php

// Declare a regular expression
$regex = "/<b>(.*)<\/b>/U";

// Declare a string
$inputString = "Name: <b>John</b> Position: <b>Developer</b>";

// Use preg_match_all() function to perform
// a global regular expression match
preg_match_all($regex, $inputString, $output);

echo $output[0][0]." <br> ".$output[0][1]."\n";

?>

Output:

John
Developer

Example 3: