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Regular Expressions

Once upon a time, testing for patterns contained within strings was an arduous process. It often

involved using string functions such as

charAt()

and

indexOf()

. Languages such as Perl imple-

mented a solution called regular expressions based on a Unix administration tools such as gred

and ed.

Regular expressions are strings with a special syntax indicating the occurrence of specific characters

or substrings within another string. These patterns range from very simple to very complicated and

can be used to do anything from removing white space to validating credit card numbers.

JavaScript has natively supported regular expressions for longer than some high-powered lan-

guages, such as Java, which only introduced native regular expression support in JDK 1.4. Entire

books are written on the subject of regular expressions because they can be very complicated; this

chapter looks specifically at how JavaScript implements regular expressions

Regular Expression Support

JavaScript supports regular expressions through the ECMAScript

RegExp

class. The constructor for

RegExp

object takes one or two arguments. The first (or only) argument is the string describing

the pattern to match; if there is a second argument, it is a string specifying additional processing

instructions.

The most basic regular expression is a regular string. For instance, to match the word

“cat”

, you

can define the regular expression like this:

var reCat = new RegExp(“cat”);

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