JavaScript: The Definitive Guide, 6th Edition

Defining Regular Expressions

In JavaScript, regular expressions are represented by RegExp objects. RegExp objects may be created with the RegExp() constructor, of course, but they are more often created using a special literal syntax. Just as string literals are specified as characters within quotation marks, regular expression literals are specified as characters within a pair of slash (/) characters. Thus, your JavaScript code may contain lines like this:

var pattern = /s$/;

This line creates a new RegExp object and assigns it to the variable pattern. This particular RegExp object matches any string that ends with the letter “s.” This regular expression could have equivalently been defined with the RegExp() constructor like this:

var pattern = new RegExp("s$");

RegExp Literals and Object Creation

Literals of primitive type, like strings and numbers, evaluate (obviously) to the same value each time they are encountered in a program. Object literals (or initializers) such as {} and [] create a new object each time they are encountered. If you write var a = [] in the body of a loop, for example, each iteration of the loop will create a new empty array.

Regular expression literals are a special case. The ECMAScript 3 specification says that a RegExp literal is converted to a RegExp object when the code is parsed, and each evaluation of the code returns the same object. The ECMAScript 5 specification reverses this and requires that each evaluation of a RegExp return a new object. IE has ...