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Using a RegExp object

This regular expression matches only the first occurrence of the word

“cat”

in a string, and it is case-

sensitive. To make the regular expression match all occurrences of

“cat”

, you can add the second argu-

ment to the constructor:

var reCat = new RegExp(“cat”, “g”);

In this line, the second argument

“g”

is short for

global

, meaning that the entire string is searching for

occurrences of

“cat”

instead of stopping after the first occurrence. If you want to make the pattern

case-insensitive, you can add the character

“i”

to the second argument (

“i”

is short for

insensitive

, as

case-insensitive

var reCat = new RegExp(“cat”, “gi”);

Some regular expression literals use Perl-style syntax:

var reCat = /cat/gi;

Regular expression literals begin with a forward slash, followed by the string pattern, followed by

another forward slash. If you want to specify additional processing instructions, such as

“g”

and

“i”

these come after the second forward slash (as in the previous example).

Using a RegExp object

After creating a

RegExp

object, you apply it to a string. You can use several methods of both

RegExp

and

String

The first thing you do with a regular expression is determine if a string matches the specified pattern.

For this simple case, the

RegExp

object has a method called

test()

, which simply returns

true

if the

given string (the only argument) matches the pattern and

false

if not:

var sToMatch = “cat”;

var reCat = /cat/;

alert(reCat.test(sToMatch)); //outputs “true”

In this example, the alert outputs

“true”

because the pattern matches the string. Even if the pattern

only occurs once in the string, it is considered a match, and

test()

returns

true

. But what if you want

access to the occurrences of the pattern? For this use case, you can use the

exec()

method.

The

RegExp exec()

method, which takes a string as an argument, returns an

Array

containing all

matches. Consider the following example:

var sToMatch = “a bat, a Cat, a fAt baT, a faT cat”;

var reAt = /at/;

var arrMatches = reAt.exec(sToMatch);

Here,

arrMatches

contains only one item: the first instance of

“at”

(which is in the word

“bat”

If you want to return all instances of

“at”

, add the

option:

var sToMatch = “a bat, a Cat, a fAt baT, a faT cat”;

var reAt = /at/g;

var arrMatches = reAt.exec(sToMatch);

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