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Regular expressions are nothing more than a sequence or pattern of characters itself. They provide the foundation for pattern-matching functionality.

Using regular expression you can search a particular string inside a another string, you can replace one string by another string and you can split a string into many chunks.

PHP offers functions specific to two sets of regular expression functions, each corresponding to a certain type of regular expression. You can use any of them based on your comfort.

POSIX Regular Expressions PERL Style Regular Expressions

POSIX Regular Expressions:The structure of a POSIX regular expression is not dissimilar to that of a typical arithmetic expression: various elements (operators) are combined to form more complex expressions.

The simplest regular expression is one that matches a single character, such as g, inside strings such as g, haggle, or bag.

Lets give explaination for few concepts being used in POSIX regular expression. After that we will introduce you with regular expression related functions.

Brackets

Brackets ([]) have a special meaning when used in the context of regular expressions. They are used to find a range of characters.

Expression Description

[0-9] It matches any decimal digit from 0 through 9.

[a-z] It matches any character from lowercase a through lowercase z.

[A-Z] It matches any character from uppercase A through uppercase Z.

[a-Z] It matches any character from lowercase a through uppercase Z.

The ranges shown above are general; you could also use the range [0-3] to match any decimal digit ranging from 0 through 3, or the range [b-v] to match any lowercase character ranging from b through v.

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Predefined Character Ranges

For your programming convenience several predefined character ranges, also known as character classes, are available. Character classes specify an entire range of characters, for example, the alphabet or an integer set:

Expression Description

[[:alpha:]] It matches any string containing alphabetic characters aA through zZ.

[[:digit:]] It matches any string containing numerical digits 0 through 9.

[[:alnum:]] It matches any string containing alphanumeric characters aA through zZ and 0 through 9.

[[:space:]] It matches any string containing a space.

PHP's Regexp POSIX FunctionsPHP currently offers seven functions for searching strings using POSIX-style regular expressions:

Function Description

ereg() The ereg() function searches a string specified by string for a string specified by pattern, returning true if the pattern is found, and false otherwise.

ereg_replace() The ereg_replace() function searches for string specified by pattern and replaces pattern with replacement if found.

eregi() The eregi() function searches throughout a string specified by pattern for a string specified by string. The search is not case sensitive.

eregi_replace() The eregi_replace() function operates exactly like ereg_replace(), except that the search for pattern in string is not case sensitive.

split() The split() function will divide a string into various elements, the boundaries of each element based on the occurrence of pattern in string.

spliti() The spliti() function operates exactly in the same manner as its sibling split(), except that it is not case sensitive.

sql_regcase() The sql_regcase() function can be thought of as a utility function, converting each character in the input parameter string into a bracketed expression containing two characters.

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Negated character classes can also be defined that match any character except those contained within the brackets. A negated character class is defined by placing a caret (^) symbol immediately after the opening bracket, like this [^abc].

PHP's Regexp PERL Compatible FunctionsPHP offers following functions for searching strings using Perl-compatible regular expressions:

Function Description

preg_match() The preg_match() function searches string for pattern, returning true if pattern exists, and false otherwise.

preg_match_all() The preg_match_all() function matches all occurrences of pattern in string.

preg_replace() The preg_replace() function operates just like ereg_replace(), except that regular expressions can be used in the pattern and replacement input parameters.

preg_split() The preg_split() function operates exactly like split(), except that regular expressions are accepted as input parameters for pattern.

preg_grep() The preg_grep() function searches all elements of input_array, returning all elements matching the regexp pattern.

preg_ quote() Quote regular expression characters

<?php$pattern="/ca[kfp]e/";$text="he was eating cake in the cafe in cape verde";if(preg_match($pattern,$text)){echo "match found <br>";}else{echo "match not found <br>";}$matches=preg_match_all($pattern,$text);echo $matches . " matches found";?>

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<?php

$pattern = "/\s/";

$replacement = "-";

$text = "Earth revolves around\nthe\tSun. welcome to world\t of php";

// Replace spaces, newlines and tabs

echo preg_replace($pattern, $replacement, $text);

echo "<br>";

// Replace only spaces

echo str_replace(" ", "-", $text);

?>

Output

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Repetition QuantifiersIn the previous section we've learnt how to match a single character in a variety of fashions. But what if you want to match on more than one character? For example, let's say you want to find out words containing one or more instances of the letter p, or words containing at least two p's, and so on. This is where quantifiers come into play. With quantifiers you can specify how many times a character in a regular expression should match.

The following table lists the various ways to quantify a particular pattern:

RegExp What it Does

p+ Matches one or more occurrences of the letter p.

p* Matches zero or more occurrences of the letter p.

p? Matches zero or one occurrences of the letter p.

p{2} Matches exactly two occurrences of the letter p.

p{2,3} Matches at least two occurrences of the letter p, but not more than three occurrences of the letter p.

p{2,} Matches two or more occurrences of the letter p.

p{,3} Matches at most three occurrences of the letter p

Examples:

Following examples will clear your concepts about matching chracters.

Expression Description

[^a-zA-Z] It matches any string not containing any of the characters ranging from a through z and A through Z.

p.p It matches any string containing p, followed by any character, in turn followed by another p.

^.{2}$ It matches any string containing exactly two characters.

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<b>(.*)</b> It matches any string enclosed within <b> and </b>.

p(hp)* It matches any string containing a p followed by zero or more instances of the sequence hp.

The regular expression in the following example will splits the string at comma, sequence of commas, whitespace, or combination thereof using the PHP preg_split() function:

<?php

$pattern = "/[\s,]+/";

$text = "My favourite colors are red, green and blue";

$parts = preg_split($pattern, $text);

// Loop through parts array and display substrings

foreach($parts as $part){

echo $part . "<br>";

}

?>

Output

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Position AnchorsThere are certain situations where you want to match at the beginning or end of a line, word, or string. To do this you can use anchors. Two common anchors are caret (^) which represent the start of the string, and the dollar ($) sign which represent the end of the string.

RegExp What it Does

^p Matches the letter p at the beginning of a line.

p$ Matches the letter p at the end of a line.

The regular expression in the following example will display only those names from the names array which start with the letter "P" using the PHP preg_grep() function and those names that end with “e”.

<?php

$pattern = "/^P/";

$pattern2="/e$/";

$names = array("Apple", "Orange", "Papaya","Peper","Pencil","Jackfruit","Guva","Brinjal");

$matches = preg_grep($pattern, $names);

$matches2=preg_grep($pattern2,$names);

// Loop through matches array and display matched names

echo "<br> the items which start with P are <br>";

foreach($matches as $match){

echo $match . "<br>";

}

echo "<br>the items which ends with e are <br>";

foreach($matches2 as $match){

echo $match . "<br>";

}

?>

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Pattern ModifiersA pattern modifier allows you to control the way a pattern match is handled. Pattern modifiers are placed directly after the regular expression, for example, if you want to search for a pattern in a case-insensitive manner, you can use the i modifier, like this: /pattern/i. The following table lists some of the most commonly used pattern modifiers.

Modifier What it Does

i Makes the match case-insensitive manner.

m Changes the behavior of ^ and $ to match against a newline boundary (i.e. start or end of each line within a multiline string), instead of a string boundary.

g Perform a global match i.e. finds all occurrences.

o Evaluates the expression only once.

s Changes the behavior of . (dot) to match all characters, including newlines.

x Allows you to use whitespace and comments within a regular expression for clarity.

The following example will show you how to perform a global case-insensitive search using the i modifier and the PHP preg_match_all() function.

<?php

$pattern = "/color/i";

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$text = "Color red is more visible than color blue in daylight.";

$matches = preg_match_all($pattern, $text, $array);

echo "from case insensitive ".$matches . " matches were found. <br>";

$pattern="/color/";

$matches = preg_match_all($pattern, $text, $array);

echo "with case sensitive ".$matches . " matches were found.<br>";

?>

Output

Word BoundariesA word boundary character ( \b) helps you search for the words that begins and/or ends with a pattern. For example, the regexp /\bcar/ matches the words beginning with the pattern car, and would match cart, carrot, or cartoon, but would not match oscar.

Similarly, the regexp /car\b/ matches the words ending with the pattern car, and would match scar, oscar, or supercar, but would not match cart. Likewise, the /\bcar\b/ matches the words beginning and ending with the pattern car, and would match only the word car.

The following example will highlight the words beginning with car in bold:

<?php

$pattern = '/\bcar\w*/';

$replacement = '<b>$0</b>';

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$text = 'Words begining with car: cart, carrot, cartoon. Words ending with car: scar, oscar, supercar.';

echo preg_replace($pattern, $replacement, $text);

?>

Output

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