There are two ways to make a RegExp object. The preferred way, as we saw in the examples, is to use a regular expression literal.

Regular expression literals are enclosed in slashes. This can be a little tricky because slash is also used as the division operator and in comments.

There are three flags that can be set on a RegExp. They are indicated by the letters g, i, and m, as listed in Table 7-1. The flags are appended directly to the end of the RegExp literal:

// Make a regular expression object that matches
// a JavaScript string.

var my_regexp = /"(?:\\.|[^\\\"])*"/g;

The other way to make a regular expression is to use the RegExp constructor. The constructor takes a string and compiles it into a RegExp object. Some care must be taken in building the string because backslashes have a somewhat different meaning in regular expressions than in string literals. It is usually necessary to double the backslashes and escape the quotes:

// Make a regular expression object that matches
// a JavaScript string.

var my_regexp = new RegExp("\"(?:\\\\.|[^\\\\\\\"])*\"", 'g');

The second parameter is a string specifying the flags. The RegExp constructor is useful when a regular expression must be generated at runtime using material that is not available to the programmer.

RegExp objects contain the properties listed in Table 7-2.

RegExp objects made by regular expression literals share a single instance:

function make_a_matcher(  ) {
    return /a/gi;
}

var x = make_a_matcher(  );
var y = make_a_matcher(  );

// Beware: x and y are the same object!

x.lastIndex = 10;

document.writeln(y.lastIndex);    // 10