RegExp

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. \d\d\d

Regular expression in JavaScript JavaScript provides regular expression capabilities through instances of the built-in RegExp object. var regTest = new RegExp(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 JavaScript 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 phoneNum = “ ”; var regTest = new RegExp(“^\\d{10}$”); if(regTest.test(phoneNum)){ 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: \d\d\d represents strings consisting of three consecutive digits. It could represents all strings containing three consecutive digits, such as “my number is 123, blah blah”. ^\d\d\d$ represents strings consisting of only three consecutive digits.

Regular expression literal It is tiresome to write duplicate backslashes. var regTest = new RegExp(“^\\d\\d\\d$”); Alternative syntax for creating a RegExp instance is var regTest = /^\d\d\d$/; 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 JavaScript 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} == \d\d\d

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. JavaScript 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*?