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Introduction to Regular Expressions in JavaScript

What exactly is a regular expression, and how is it handled in JavaScript? These are the questions tackled by this five-part series. In this first part, we'll introduce the topic of regular expressions (often abbreviated RegExp), and delve into objects, patterns, and variables.

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By: Chrysanthus Forcha
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July 08, 2009
  1. · Introduction to Regular Expressions in JavaScript
  2. · Simple Word Matching
  3. · Meaning of Pattern
  4. · Regular Expression Object
  5. · Simple Usage of the Literal Text Format and the Constructor Function
  6. · The Flags

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Introduction to Regular Expressions in JavaScript
(Page 1 of 6 )


Consider the string, 

"This is a man".

Assume that you do not know the content of the string; the string might have been typed by the user and the JavaScript code has assigned it to a variable. You may have the following two questions:

  1. Does the sting have the word "man"?

  2. If it the string has the word "man," can you change it to "woman"?

There are many other questions that are similar (and rather complex) to the above two questions. Handling this in code is a subject called Regular Expressions, abbreviated RegExp in JavaScript.

The Word RegExp

In the above example, "man" is a RegExp. More generally, a RegExp is a string (usually a small string) of characters for which you want to know whether it exists in some available string. This available string might have been assigned to a variable.


When the RegExp is seen in the available string, we say matching has occurred. That is, the RegExp has matched the string. When matching occurs, replacement can follow. If the regExp "man" in the above example is seen, it can be replaced by the word "woman."

Modern and Old-Fashioned Ways of Coding RegExps

At first, to answer the above types of questions, you had to do the coding using programming basics (declaration of variable, conditions, loops, etc). These questions can be classified (grouped). The computer language Perl came up with functions to handle the above questions, which gives the programmer less work.

The programmer uses these functions in a special way without really being conscious that he is using them. The use of these inbuilt functions is made convenient with special symbols. I do not know the inventor of Regular Expressions, but JavaScript modeled its Regular Expressions from Perl.

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