↑
Main Page

Backreferences

Expressions to match leading and trailing white space are very simple thanks to the

character class

that matches all the white space characters:

var reExtraSpace = /^\s+(.*?)\s+$/;

This regular expression looks for one or more occurrences of white space at the beginning of the string,

followed by any number of additional characters (which are captured in a group), followed by one or

more occurrences of white space at the end of the string. By using this in conjunction with the

String

object’s

replace()

method and backreferences, you can define your own

trim()

method:

String.prototype.trim = function () {

var reExtraSpace = /^\s+(.*?)\s+$/;

return this.replace(reExtraSpace, “$1”);

};

With this method, you can create trimmed versions of strings very easily:

var sTest = “ this is a test “;

alert(“[“ + sTest + “]”); //outputs “ [ this is a test ] “

alert(“[“ + sTest.trim() + “]”); //outputs “ [this is a test]”

Backreferences

So what do you do with groups after the expression has been evaluated? Each group is stored in a spe-

cial location for later use. These special values, stored from your groups, are called

backreferences

Backreferences are created and numbered by the order in which opening parenthesis characters are

encountered going from left to right. For example, the expression

(A?(B? (c?)))

creates three back-

references numbered 1 through 3:

(A? (B? (c?)))

(B? (c?))

(c?)

The backreferences can then be used in a couple of different ways.

First, the values of the backreferences can be obtained from the

RegExp

constructor itself by using the

test()

match()

, or

search()

methods. For example:

var sToMatch = “#123456789”;

var reNumbers = /#(\d+)/;

reNumbers.test(sToMatch);

alert(RegExp.$1); //outputs “123456789”

This example tries to match the pound sign followed by one or more digits. The digits are grouped so

they will be stored. After the

test()

method is called, all backreferences have been stored on the

RegExp

constructor starting with

RegExp.$1

, which stores the first backreference (it continues with

RegExp.$2

if there is a second,

RegExp.$3

if there is a third, and so on). Because the group matches

“123456789”

, that is what is stored in

RegExp.$1

206

Chapter 7

10_579088 ch07.qxd 3/28/05 11:38 AM Page 206