In many cases, developers run into problems whenever data is collected in free text fields, as most programming deals with data entry. Regular expressions are used almost everywhere in modern application programming.
- Regular expressions help in validating text strings that are of interest to the programmer.
- It offers a powerful tool for parsing, pattern matching and changing text data.
- It helps in finding a specific string pattern and flexibly retrieving matching results.
- It helps in parsing text files looking for a specific sequence of characters for further analysis or data manipulation.
- The built-in regular expression functions provide simple and straightforward solutions for identifying patterns.
- This effectively saves a lot of development time looking for a particular string pattern.
- It helps in validating important user information such as email address, phone numbers and IP address.
- It helps in highlighting special key words in a file based on search or input results.
- It helps in identifying certain template tags and replacing that data with actual data as required.
- Regular expressions are very useful for building an HTML templating system. recognizing tags.
- Regular expressions are mainly used for browser detection, spam filtering, password strength checking, and form validation.
- Regular expressions are case sensitive by default.
- There is a difference between strings inside single quotes and strings inside double quotes in PHP. The former are treated literally, whereas for strings within double quotes means that the contents of the variable are printed instead of just typing their names.
// Declare a regular expression $regex = ’/ ^ [a-zA-Z] * $/’ ;
// Declare the line $nameString = ’Sharukh khan’ ;
// Use preg_match() function for
// search for a 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:
// Declare a regular expression $regex = "/ ( . *) / U " ;
// Declare the line $inputString = "Name: John Position: Developer " ;
// Use the preg_match_all() function to execute
// global regular expression match preg_match_all ( $regex , $inputString , $output ); echo $output [0] [0]. "
" . $output [0] [1]. " " ;
?> Output:
John Developer
Example 3:
// Declare a regular expression $regex = "([0-9] +)" ;
// Declare the line $original = "Completed graduation in 2004" ; $replaceWith = "2002" ;
// Use the ereg_replace() function to search
// string template on another line $original = ereg_replace ( $regex , $replaceWith , $original );
// Show result echo $original ;
?> Output: Completed graduation in 2002
Example 4:
// Declare the string $ip = " 134.645.478.670 " ;
// Declare regular expression $regex = "/./" ;
// Use preg_split() function for
// convert this string to
// array $output = preg_split ( $regex , $ip ); echo " $output [0]
" ; echo "$output [1]
" ; echo "$output [2]
" ; echo "$output [3]
" ;
?> Output:
134 645 478 670
Metacharacters:Regular expressions use two types of characters: regular characters and metacharacters. Regular characters - these are characters that have a "literal" meaning, and the metacharacters - these are the characters that have a "special" meaning in the regular expression. Metacharacter Description Example . It matches any single character other than a new line. /./ matches string which has a single character. < / tr> ^ It matches the beginning of string. / ^ engineer / matches any string that starts with engineer. $ It matches the string pattern at the end of the string. / com $/ matches string ending with com for example google.com etc. * It matches zero or more characters. / com * / matches commute, computer, compromise etc. + It matches preceding character appear atleast once. For example / z + oom / matches zoom. It is used to esacape metacharacters in regex. /google.com/ will treat the dot as a literal value, not metacharacter. az It matches lower case letters. engineer AZ It matches upper case letters. GEEKS 0-9 It matches any number between 0 and 9. / 0-5 / matches 0, 1, 2, 3, 4, 5 […] It matches character class. / [pqr] / matches pqr Other examples: Regular expression Meaning ^ [.-a-z0-9A-Z ] It matches string with dot, dash and any lower case letters, numbers between 0 and 9 and upper case letters. + @ [a-z0-9A-Z] It matches string with @ symbol in the beggining followed by any lower case letters, numbers between 0 and 9 and upper case letters. +. [Az] {2, 6} $/ It escapes the dot and then matches string with any lower case letters with string length between 2 and 6 at the end. Notes : - Metacharacters are very effective in solving regular expression pattern matching. It handles a lot of complex patterning.
- Every character that is not a metacharacter is definitely a regular character.
- Every regular character corresponds to the same character.
POSIX Regular Expressions:Some regular expressions in PHP are similar to arithmetic expressions called POSIX regular expressions. Sometimes complex expressions are created by combining different elements or operators into regular expressions. The most basic regular expression - this is the one that matches one character. Let’s take a look at some POSIX regular expressions. Regex Meaning < / th> [0-9] It matches digit from 0 through 9. [az] It matches any lowercase letter from a through z. [AZ] It matches any uppercase letter from A through Z. [aZ] It matches any lowercase letter a through uppercase letter Z. [: lower: ] It matches any lower case letters. [: upper:] It matches any upper case letters. [: alpha:] It matches all alphabetic characters or letters from az and AZ. [[: alpha:]] It matches any string containing alphabetic characters or letters. [: alnum:] It matches all alphanumeric characters ie all digits (0-9) and letters (az AZ). [[: alnum:]] It matches any string containing alphanumeric characters and digits. [: digit:] It matches all the digits from 0 to 9. [[: digit:]] It matches any string containing digits from 0 to 9. [: xdigit:] It matches all the hexadecimal digits. [: punct: ] It matches all the punctuation symbols. [: blank:] It matches blank characters like space and tab. [:space: ] It matches all whitespace characters like line breaks. [ [: space:]] It matches any string containing a space. [: cntrl:] It matches all control characters. [: graph:] It matches all visible or printed characters other than spaces and control characters. [: print:] It matches all printed characters and spaces other than control characters. [: word:] It matches all word characters like digits, letters and underscore. Quantifiers in regular you rage.Quantifiers - they are special characters that indicate the number, frequency, or number of occurrences, or the occurrence of a parenthesized character or group of characters. They are also called greedy and lazy expressions. Let’s take a look at some concepts and examples of quantifiers. Quantifier Meaning a + It matches the string containing at least one a. a * It matches the string containing zero or more a’s. a? It matches any string containing zero or one a’s. a {x} It matches letter ’a’ exaclty x times. a {2, 3} It matches any string containing the occurrence of two or three a’s. a {2,} It matches any string containing the occurrence of at least two a’s. a {2} It matches any string containing the occurrence of exactly two a’s. a {, y} It matches any string containing the occurrence of not more than y number of a’s. a $ It matches any string with ’a’ at the end of it. ^ a It matches any string with ’a’ at the beginning of it. [^ a-zA-Z] It matches any string pattern not having characters from a to z and A to Z. aa It matches any string pattern containing a, then any character and then another a. ^. {3} $ It matches any string having exactly three characters. < / table> Notes : - The $character inside the expression will match the end of the target string.
- The characters *,?, + In regular expression denote the frequency of occurrence of the symbol. If this happens zero or more times, zero or one times, and one or more times.
- The ^ character inside the expression will match the beginning of the target line.
- . the metacharacter matches any single character other than a newline.
Well-known regexp engines: - Regexp
- RegexBuddy
Conclusion.Regular expression - it is a pattern that describes some text in a string in a particular pattern, or is defined as a pattern matching algorithm expressed in a single string. Regular expressions are very useful in the programming world for validating and recognizing certain patterns. PHP provides many built-in functions that support regular expressions. Metacharacters help you create complex patterns.
