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Regular ExpressionsHow will you know it’s them when you see them?111Regular expressions are common features in computerlanguages and text editors. Learning how to use them willincrease your performance as a computer programmer andproblem solver. JavaScript was designed with regex in mind.The phrase Regular Expression can be shortened. Regular expressions are commonly referredto as regex, or regexp in conversation. We use regular expressions to find or match a characterpattern within a word or a line of text. For example, we can search for the word clock in theword clockwork. We can also find the words lock and work in the same word.lockclockworkRegular expressions helps us find words or character patterns inside text and then store theresults in an array for further manipulation or modification. Another important function ofregular expressions lies in their ability to count the number of times a pattern repeats inside aword or a sentence. For example, there are two o’s in the word clockwork. The following regularexperssion example demonstrates how we can find and count all o’s in the word clockwork:var pattern = /o/g;var result = “clockwork”.match(pattern);alert(result.count);// g modifier stands for global search// match is a member function of string object// display how many times “o” was foundThe actual regular expression pattern is stored between two / symbols. But they are not exactlythe same as double quotes when defining a string. The first / explicitly indicates the start of thepattern whereas the second / explicitly indicates end. It usually matches end of line character.
112As you may know an object can contain methods. Remember that everything in JavaScript isan object. Even a literal string specified with double quotes. In this case, JavaScript has alreadyprovided such a method (called match) to execute a regular expression match:“clockwork“.match(/o/g) ......... Regex match - returns array: [“o”, “o”].“clockwork”.toUpperCase()....... Returns “CLOCKWORK“.“clockwork”.charAt(5) ........... Returns “w” because index counting starts from 0.“clockwork”.length .............. Returns 9 but this is not an object method,this is a member variable or object property.There are a few other methods. In the context of this section we are only interested in themethod called match (also known as object’s member function). The example above shows thatmatch is not a unique method of the string’s object. And it also shows that it is possible to accessit directly from the string definition.As you can see we can call all these functions directly after the closing double quote by accessingthe string’s member function with the dot operator. This can be done without having tostore the string within a unique variable first, but by calling it directly on the value itself.By the way, doing it this way still creates a variable in memory and the value “clockwork” is assignedto it. It’s just we won’t have a name handle to refer to it once the code is finished executing.Sometimes this makes the code cleaner and easier to read.You see, JavaScript already has a number of member functions to help us work with strings.The function match is not the only one, but that is the one we need in order to execute a regularexpression pattern match.Remember that this is rather a less common way to execute a command on a string. This is becausewe are defining the string “clockwork” on the fly and simultaneously accessing its memberfunction match. But the exact same thing could be done using the following statements:var text = “clockwork”;var result = text.match(/o/g);The difference is that here we first stored the string “clockwork” in a variable and only thencalled the function match. But sometimes it is not necessary to do it this way. The choice belongsto the programmer and the circumstances in which the code is written.What else can a regular expression match?Besides matching the characters from the alphabet we can also match other entities. For examplespecial characters like #, $, % and others. In addition, regular expressions make it possiblematching character ranges like A-Z or a-z (notice that the letter case matters) and also
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