Defining Regular Expression in JavaScript

Introduction

The Regular Expression object is used to store the search pattern.We define a RegExp object with the new keyword.

Implementation

The following code line defines a RegExp object called patt1 with the pattern "e":

var patt1=new RegExp("e");

 When you use this RegExp object to search in a string, you will find the letter "e".

Methods of the RegExp Object

The RegExp Object has 3 methods,

1. test()
2. exec()
3. compile()

 test() method

The test() method searches a string for a specified value. Returns true or false

var patt1=new RegExp("e")   document.write(patt1.test("The best things in life are free"));

 Since there is an "e" in the string, the output of the code above will be:

true

exec() Method

The exec() method searches a string for a specified value. Returns the text of the found value. If no match is found, it returns null

Example 1

var patt1=new RegExp("e");    document.write(patt1.exec("The best things in life are free")); 

 Since there is an "e" in the string, the output of the code above will be:

e

You can add a second parameter to the RegExp object, to specify your search. For example; if you want to find all occurrences of a character, you can use the "g" parameter ("global"). 

When using the "g" parameter, the exec() method works like this:

• Finds the first occurence of "e", and stores its position
• If you run exec() again, it starts at the stored position, and finds the next occurence of "e", and stores its position

Example 2

var patt1=new RegExp("e","g");    
do    
{    
result=patt1.exec("The best things in life are free");    
document.write(result);    
}while (result!=null)

 Since there is six "e" letters in the string, the output of the code above will be:

eeeeeenull

compile() Method

The compile() method is used to change the RegExp.compile() can change both the search pattern, and add or remove the second parameter.

Example

var patt1=new RegExp("e");
document.write(patt1.test("The best things in life are free"));   
patt1.compile("d");
document.write(patt1.test("The best things in life are free"));

 Since there is an "e" in the string, but not a "d", the output of the code above will be:

truefalse

Conclusion

In this I have explain the use of RegExp object in JavaScript. There are 3 key methods to use in real world scenarios. You can find examples in my article.