JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

and contains any special flags for that expression. The two previous examples could
equivalently be declared as
var pattern = new RegExp("http");
var patternIgnoringCase = new RegExp("http", "i");
The constructor syntax is most commonly used when the pattern to match against is not
determined until runtime. You might allow the user to enter a regular expression and then pass
the string containing that expression to the RegExp() constructor.
The most basic method provided by the RegExp object is test(). This method returns a
Boolean indicating whether the string given as its argument matches the pattern. For example,
we could test
var pattern = new RegExp("http");
pattern.test("HTTP://WWW.W3C.ORG/");
which returns false because pattern matches only strings containing ―http.‖ Or we could test
using the case-insensitive pattern,
var patternIgnoringCase = new RegExp("http", "i");
patternIgnoringCase.test("HTTP://WWW.W3C.ORG/");
which returns true because it matches for strings containing ―http‖ while ignoring case. Of
course, you won't see much unless you use the returned value:
var patternIgnoringCase = new RegExp("http", "i");
alert(patternIgnoringCase.test("HTTP://WWW.W3C.ORG/"));
Because of JavaScript's automatic type conversion, you can invoke RegExp methods on
regular expression literals (just like String methods on string literals). For example,
alert(/http/i.test("HTTP://WWW.W3C.ORG/"));
would alert out true as well.
Creating Patterns
The example patterns so far merely check for the presence of a particular substring; they
exhibit none of the powerful capabilities to which we have alluded. Regular expressions use
special character sequences enabling the programmer to create more complicated patterns.
For example, special characters provide a way to indicate that a certain character or set of
characters should be repeated a certain number of times or that the string must not contain a
certain character.
Positional Indicators
The first set of special characters can be thought of as positional indicators, characters that
mandate the required position of the pattern in the strings against which it will be matched.