Lecture 12 Regular Expression in Java Script Reg

Lecture 12: Regular Expression in Java. Script Reg. Exp

Regular Expression �A regular expression is a certain way to describe a pattern of characters. �Pattern-matching or keyword search. �Regular expressions are frequently used to test whether or not a string entered in an HTML form has a certain format. �For example, you want to test if a user entered a string that contains 3 consecutive digits. �ddd

Regular expression in Java. Script �Java. Script provides regular expression capabilities through instances of the built-in Reg. Exp object. �var reg. Test = new Reg. Exp(pattern, modifiers); �Pattern specifies the patthern of an expression �Modifiers specify if a search should be global, case-senstive �The i modifier is used to perform case-insensitive matching �The g modifier is used to perform a global match (find all matches rather than stopping after the first match) �match() is a method of a String instance returns true if its string matches the pattern; otherwise, returns false.

macth() in Java. Script �match() is a method of a String instance returns true if its string matches the pattern; otherwise, returns false. For example: var word = “I am a FSU student”; var re = /fsu/i; if(word. match(re)) alert(“Yes”); else alert(“No”);

test() � test(argument) is a method of a Regular Expression instance returns true if the argument contains the pattern; otherwise, returns false. for example: var phone. Num = “ 8502349 023”; var reg. Test = new Reg. Exp(“^\d{10}$”); if(reg. Test. test(phone. Num)){ alert(“Valid phone number”); } else{ alert(“Invalid phone number”); } What is the output?

Caret ^ and dollar sign $ �^ indicates the beginning of a string �$ indicates the end of a string �For example: �ddd represents strings consisting of three consecutive digits. It could represents all strings containing three consecutive digits, such as “my number is 123, blah”. �^ddd$ represents strings consisting of only three consecutive digits.

Regular expression literal �It is tiresome to write duplicate backslashes. � var reg. Test = new Reg. Exp(“^\d\d\d$”); �Alternative syntax for creating a Reg. Exp instance is � var reg. Test = /^ddd$/; �The expression on the right-hand side is known as regular expression literal. The scripting engine automatically escaping any backslash characters contained in the regular expression literal.

Special characters �The simplest form of regular expression is a character that is not one of the regular expression special characters: �^ $ . * + ? ( ) [ ] { } | �A special character is escaped by preceding it with a backslash. For example, $$ represents the set of strings that end with a dollar sign. �A. means a character except for a line terminator, such as n and tab. �* is called Kleene star, represents infinitely large sets.

Escape Code �A escape code regular expression represents multiple characters �A list of escape code � d – digits : 0 through 9 � D – any character except those matched by d � s – space: any Java. Script white space or line terminator ( space, tab, line feed, etc) � S – any character except those matched by s � w – “word” character: any letter (a through z and A through Z), digit , or underscore � W – any character except those matched by W

White space in regular expression �A white space is significant within a regular expression �For example, we have a regular expression, such as ^d. w$ � Does “ 3. A” match this regular expression? � Does “ 9. B” match this regular expression?

Concatenation Operator �Simple regular expressions can be composed into more complex regular expressions using concatenation operator. � For instance: ^d s$ is a concatenated regular expression �When you want to concatenate a regular expression with itself multiple times, you can use the quantifier shorthand notation. � d{3} == ddd

Union Operator �Union operator is represented by the pipe symbol | �For example, d|s represents the set consisting of all digit and white space characters. �Concatenation operator takes precedence over union, so +|-d|s consists of +, the two-character strings beginning with – followed by a digit, and the white space characters.

Character class �It is tedious to use the union operator to represent a set of all lowercase letters. �Java. Script provides a character class that can be used for such purpose. � [a-z] � [A-Z] � [0 -9] �How to represent w using character class?

Examples of regular expression �What does d{3, 6} represents? �How about (+|-){0, 1}d? �How about d*?