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Instance properties

already been discussed in this chapter. But the

RegExp

object also has properties, both on the constructor

and on instances of

RegExp

. Both sets of properties change as patterns are created and tested.

Instance properties

An instance of

RegExp

has a number of properties that can be of use to developers:

global

— A Boolean value indicating whether or not the

option has been set

ignoreCase

— A Boolean value indicating whether or not the

option has been set

lastIndex

— An integer representing the character position where the next match will be

attempted (only filled after using

exec()

test()

, otherwise is 0)

multiline

— A Boolean value indicating whether the

option has been set

source

— The string source of the regular expression. For example, the expression

/[ba]*/

returns

“[ba]*”

as its

source

You don’t typically use the

global

ignoreCase

multiline

, and

source

properties because you often

already have the data they provide:

var reTest = /[ba]*/i;

alert(reTest.global); //outputs “false”

alert(reTest.ignoreCase); //outputs “true”

alert(reTest.multiline); //outputs “false”

alert(reTest.source); //outputs “[ba]*”

The really useful property is

lastIndex

, which tells you how far the regular expression has traveled

along a string before stopping:

var sToMatch = “bbq is short for barbecue”;

var reB = /b/g;

reB.exec(sToMatch);

alert(reB.lastIndex); //outputs “1”

reB.exec(sToMatch);

alert(reB.lastIndex); //outputs “2”

reB.exec(sToMatch);

alert(reB.lastIndex); //outputs “18”

reB.exec(sToMatch);

alert(reB.lastIndex); //outputs “21”

In this example, the regular expression

reB

is looking for the character

. When it is first executed

against

sToMatch

, it finds the

in the first position, position 0; therefore, the

lastIndex

property is set

to 1, which is where the matching picks up when

exec()

is called again. When it’s called again, the

expression finds the

in position 1, which sets

lastIndex

to 2. When called a third time, it finds the

in position 17, setting

lastIndex

to 18, and so on.

If you want the matching to start at the beginning again, you can always set

lastIndex

to 0:

var sToMatch = “bbq is short for barbecue”;

var reB = /b/g;

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Regular Expressions

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