vb.net barcode Putting Off Learning RegExp Syntax in Java

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Putting Off Learning RegExp Syntax
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Even though I will cover RegExp objects more fully in 5, if you are new to both JavaScript and programming, I suggest simply passing string parameters to the four methods that work with RegExp objects. replace() and split() work with either a string or a RegExp parameter. So, a string will do as is. match() and search() work only with a RegExp parameter, but JavaScript implicitly converts a string parameter to a RegExp object. So, just as a string is converted to a wrapper object by passing it to new and String(), it can be converted to a RegExp object by passing it to new and RegExp(). Insofar as JavaScript does the latter just as quietly as the former, this means that a beginner can put off learning RegExp syntax until after learning JavaScript syntax. I recommend you do.
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To illustrate this beginner-friendly JavaScript string to RegExp conversion feature, double-clear Firebug, and then enter and run the following sample. As Figure 2 29 displays, passing a string to match() and search() works just dandy: var incredibles = "Mr. Incredible, Elastigirl, Violet, Dash, Jack-Jack"; incredibles.match("Jack"); // ["Jack"] incredibles.search("Jack"); // 42
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Figure 2 29. JavaScript quietly converts "Jack" to /Jack/ for both match() and search(). JavaScript quietly passed "Jack" to RegExp(), which like String() is referred to as a constructor function. So to explicitly do what JavaScript implicitly did, let s enter and run the following in Firebug. As Figure 2 30 displays, the return values are the same: var incredibles = "Mr. Incredible, Elastigirl, Violet, Dash, Jack-Jack"; incredibles.match(new RegExp("Jack")); // ["Jack"] incredibles.search(new RegExp("Jack")); // 42
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Figure 2 30. Passing a RegExp for a literal string to match() and search() Note, however, that, when JavaScript converts a string to a RegExp object, the g, i, and m flags, which we ll explore in 5, are not set. There s no way for JavaScript to save the day if we intended to pass /jack/ig but instead passed "jack" to match(), as Figure 2 31 displays: var incredibles = "Mr. Incredible, Elastigirl, Violet, Dash, Jack-Jack"; incredibles.match(/jack/ig); // ["Jack", "Jack"] incredibles.match("jack"); // null Note that match() conveys failure, which is to say no array of matching strings, by returning null. Remember from 1 that null conveys no value on the heap, in other words, no object, array, or function. That is why match() returned null instead of undefined.
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Figure 2 31. JavaScript does not set the i, g, or m flag when converting a string to a RegExp object.
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In this chapter, you learned that JavaScript converts a string to a wrapper object whenever the string is the left operand to the . or [] operator, quietly passing the string to the String() constructor and then just as quietly reverting the wrapper to a string by invoking its valueOf() method. String wrappers manipulate characters as if they were read-only elements. In other words, string values are immutable, and wrapper methods return a new value without changing the original. So, typically you will want to save the return value or immediately pass it to one of the operators we ll explore in 3. Most JavaScript operators are particular about the value type of their operands. Knowing this, JavaScript will save the day by converting an operand of the wrong type to the correct one. Though JavaScript does so behind the scenes, we explored how to do so by passing values to String(), Number(), Boolean(), and Object(). If invoked without the new operator, those constructors convert their argument to a string, number, boolean, or object. So, a string can be converted to a wrapper by passing it to Object() without new or to String() with new: Object("Spider-Man"); new String("Spider-Man"); So, when JavaScript creates a wrapper for a string, it s really converting the value from the string type to the object type. The same thing goes for number or boolean wrappers, too. Therefore, value type conversion is vital for manipulating values with wrapper methods or with operators. You ll learn more about the latter in 3. Take a breather, and I ll see you there!
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