javascript

Chapter 5: Reference Types
Pattern Limitations
Although ECMAScript ’ s regular-expression support is fully developed, it does lack some of the advanced
regular-expression features available in languages such as Perl. The following features are not supported
in ECMAScript regular expressions (for more information, see www.regular - expressions.info ):
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The \A and \Z anchors (matching the start or end of a string, respectively)
Lookbehinds
Union and intersection classes
Atomic grouping
Unicode support (except for matching a single character at a time)
Named capturing groups
The s (single- line) and x (free- spacing) matching modes
Conditionals
Regular - expression comments
Despite these limitations, ECMAScript ’ s regular - expression support is powerful enough for doing most
pattern - matching tasks.
The Function Type
Some of the most interesting parts of ECMAScript are its functions, primarily because functions actually
are objects. Each function is an instance of the Function type that has properties and methods just like
any other reference type. Because functions are objects, function names are simply pointers to function
objects and are not necessarily tied to the function itself. Functions are typically defined using function -
declaration syntax, as in this example:
function sum (num1, num2) {
return num1 + num2;
}
This is almost exactly equivalent to using a function expression, such as this:
var sum = function(num1, num2){
return num1 + num2;
};
In this code, a variable sum is defined and initialized to be a function. Note that there is no name
included after the function keyword because it ’ s not needed — the function can be referenced by the
variable sum . Also note that there is a semicolon after the end of the function, just as there would be after
any variable initialization.
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