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| == Introduction ==
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| Regular expressions are a key to powerful ,flexible and efficient processing .Regular
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| expressions are themselves,with a general pattern notation ,like a mini programming language
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| that allows you to describe and parse text.With additional support provided by the particular tool being used,
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| regular expressions can add,remove,isolate and generally fold,spindle and mutilate all kinds
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| of text and data.It might be as simple as a text editor's search command or as powerful
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| as a full text processing language.They are a specific kind of text pattern that can be used with many modern
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| applications and programming languages.They can be used to verify whether input fits into the text pattern,to find
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| text that matches the pattern within a larger body of text,to replace text matching the pattern with other text or
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| rearranged bits of the matched textand to split a block of text into a list of subtexts.
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| == ''Overview'' ==
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| Regular expressions and the way they are used can vary wildly from tool to tool.When looking at regular expressions
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| in the context of their host language or tool,there are three broad issues to consider:
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| 1)what metacharacters are supported and their meaning.Often known as the regex flavor.
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| 2)how Regular expressions interface with the language or tool,such as how to specify regular expression operations,what
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| operations are allowed and what text they operate on.
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| 3)How the regular expression engine actually goes about applying a regular expression to some text.the method that
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| the language or tool designer uses to implement the regular expression engine has a strong influence on the results one
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| might expect from any given regular expression.
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| ==
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| Importing the Regular expression library ==
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| Some of the programmng languages like Ruby have the built in support for regular expressions.Hence there is no need to do
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| anything to import.While some languages like c# ,java etc require to import a library by writing an import statement
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| in the source code.But there are some languages not having any support for regular expression.For these languages,
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| it is required to compile an link in the regular expression support by the programmer.
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| JavaScript
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| Regular expression support is built in.
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| == Ruby ==
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| Regular Expression support is built in.
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| == Python ==
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| import re
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| It is required to import the re module into the script.Only then,Python's regular expression functions can be used.
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| == Perl ==
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| Regular Expression support is built in.
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| == PHP ==
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| The preg functions are built in and avaulable in PHP 4.2.0 and later.
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| == C# ==
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| using System.Text.RegularExpressions;
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| == VB.Net ==
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| Imports System.Text.RegularExpressions
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| == Java ==
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| import java.util.regex.*;
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| == Creating Regular Expression Objects ==
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| The regular expression has to be compild first before the regular expression engine can match a regular expression
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| to a string.THis happens at the time when the application is running during which the regular expression constructor
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| parses the string holding the regular expression.The string is then converted into a tree structure or a state machine.
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| This tree is then traversed by the function performing the match of actual pattern .The programming languages having
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| support for literal regular expressions copile the code when execution reaches the regular expression operator.
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| == Ruby ==
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| In Ruby,a special syntax is used to declare regular expressions.A regular expression s placed between two forward
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| slashes.And if there is a forward slash within the regular expression,that is escaped using a backslash.If we don't
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| want to escape forward slashes ,we can prefix it with a %r and then using any puctuation character a the delimiter.
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| Ruby is quite smilar to Java Script.The only difference is that the name of the class is Regexp as one word in Ruby
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| an is RegExp with camel caps in JavaScript.
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| myregexp = /regex pattern/;
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| Regular expression retrieved from user input ,as a string stored in the variable userinput:
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| myregexp=Regexp.new(userinput);
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| ==
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| PERL ==
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| The data processing in Perl relies heavily on regular expressions.The literal regular expressions are used with the
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| pattern-matching operator and the substitution operator.The pattern matching operator starts with m and contains two
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| forward slashes with the required regex between them.Forward slashes should be escaped with the backslash.While using
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| any type of opening or closing punctuation(parentheses,braces or brackets) as a delimiter,they must be matched up.Ex-
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| m{regex}.Using any other puctuation requires the writing of that same character twice.
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| The substitution operator starts with s.If we are using brackets or similar puctuation as the delimiter,
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| we need to have two pairs:s[regex][replace].For rest of the puctuation ,it should be used three times:s/regex/replace/.
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| Perl clearly differentiates between dollars used as anchors and dollars used for variabe interpolation.In Perl, @ sign is
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| used for variable interpolation .It should be escaped in literal regular expressions in Perl code.
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| to compile a regular expression ,"quote regex" operator can be ued and assigned to a variable.The same syntax
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| as match operator is used except that it starts with qr instead of m.
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| $myregex = qr/regex pattern/
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| The REgular expression is retrieved from the user input ,and stored as a string in the variable $userinput:
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| $myregex=qr/$userinput/
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| ==
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| PHP
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| ==
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| The three regex engines in PHP are the "preg","ereg" and "mb_ereg" engines.PHP does not have a native regular expresson
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| type unlike Java Sript and Perl.The regular expressions are required to be quoted as strings.Within the given string,
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| the regular expression should be quoted as a Perl style literal regular expression.For ex-while writing /regex/ in
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| Perl,in PHP the string becomes '/regex/'.PHP does support both single -quoted and double quoted strings.
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| But PHP does not provide a way to store a compiled regular expression into a variable.So,it has to be passed a
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| string to one of preg functions.
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| == Python ==
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| In Python,the literal regular expressions are required to be passed as strings.There are various ways to quote strings
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| provided by Python ,depending on the characters.The different ways of quoting may reduce the number of characters needed
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| to escape with backslashes.The raw strings in Python don't require to escape any characters.For ex-r"\d+" instead of
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| "\\d+".But the raw strings cannot be used when there are both single and double quoted strings in our regular expression.
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| In such a case,the raw string can be triple quoted.
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| reobj=re.compile("regex pattern")
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| The Regular Expression retrieved from user input,as a string stored in the variable userinput:
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| reobj = re.copile(userinput)
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| == JavaScript ==
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| To use the same object again,it can be assigned to a variable.If it is stored in a string variable,the RegExp() constructor
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| to compile the regular expression can be used.
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| ==
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| Java ==
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| The objects can be created with the Pattern.compile() class factory.Creation requires only a parameter:a string with
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| a regular expression.If there is a syntax error,the Pattern.compile() factory throws a Pattern SyntaxException.
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