Java Script : Regular Expressions Overview

Definition

In JavaScript , a regular expression is an object that describes a pattern of characters.

Commonly are used in functions for search o replace specific text , or pattern-matching (values validate)

Syntax

A regular expression basically is formed by : Pattern and Modifier

Pattern : specifies the pattern of an expression
Modifier: specify if a search should be global, case-sensitive, etc.

Modifiers can be:

Modifier	Description
i	Perform case-insensitive matching
g	Perform a global match (find all matches rather than stopping after the first match)
m	Perform multiline matchin

You can write a regular expression in Javascript in two modes:

use RegExp class : 

var patt=new RegExp(pattern,modifiers);

or more simply (used for all examples in this article):

var patt=/pattern/modifiers;

Example:

• Do global search of "hello" word: 


var regExp = /hello/gi;

Where "hello" is a pattern and "g" and "i" are modifiers

Complete implementation can be:

function myFunction()
{
var str = "Hello , some body in home ? ";
var regExp = /hello/ig;
var result = str.match(regExp);
alert(result);
}


A regular expression pattern can be conformed by :

Brackets

Brackets are used to find a range of characters

Example  :

• Do global (modifier "g") search of number between 1 an 4 :

• var regExp = /[1-4]/g;
• Example: From value "9043192567" , the match is "43125"

Do global (modifier "g") search of any character from lowercase "a" to lowercase "z":

• var regExp = /[a-z]/g;
• Example: From value "HELLO kitty", the match value is "kitty"

Metacharacters

Metacharacters are characters with special meaning

Example:

• Find a white space character:
• var regExp = /\s/g;

• Find a digit :
• var regExp = /\d/g;

Quantifiers

Quantifiers match the preceding subpattern a certain number of times. The subpattern can be a single character, an escape sequence, a pattern enclosed by parentheses or a character set.

Example :

• Match exactly all value in some text where "o" character appear 2 times :
• var regExp = /o{2}/g
• Example value :  "tooth and nose" , the match value is "oo" from "tooth" word

• Matches any digit that is not followed by "-" character  :
• var regExp = /\d(?!-)/g
• Example value :  "01-800-123-456" , the match value is 0,8,0,1,2,456

RegExp Object Method

In Javascript ,  RegExp object provides  following methods :

• exec() : Tests for a match in a string. Returns the first match

1. function myFunction() {
2.     var str = "The best things in life are free";
3.     var patt = new RegExp("e");
4.     var res = patt.exec(str);
5.     alert(res);
6. }
• The result of  res will be : "e"

• test() : Tests for a match in a string. Returns true or false

1. function myFunction() {
2.     var str = "The best things in life are free";
3.     var patt = new RegExp("e");
4.     var res = patt.exec(str);
5.     alert(res);
6. }

• The result of  res will be : true

• tostring(): Returns the string value of the regular expression

1. function myFunction() {
2.     var patt = new RegExp("Hello World", "g");
3.     var res = patt.toString();
4.     alert(res);
5. }

• The result of  res will be : "/Hello World/g"

Common Examples:

• Email Validation: Boolean function it return true if e-mail is a valid  adress, return false if isn´t.

1. function emailValidation(email)
2. {
3.  var regExp = new RegExp("^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}$","g");
4.  alert(regExp.test(email));
5. }

• Print Pages Validation: Boolean function it return true if received value match with the example values : 1,2,3-4,1 (like when you send to print pages)

1. function printPagesValidation(pages)
2. {
3.  var regExp = new RegExp("^([\\d]+)$|([\\d]+)[-,]([\\d]+)$","g");
4.  alert(regExp.test(pages));
5. }

References :

JavaScript RegExp Object : http://www.w3schools.com/jsref/jsref_obj_regexp.asp
Regular Expressions patterns: http://www.javascriptkit.com/javatutors/redev2.shtml