Hi,
We all know about the Regular expression and its uses and how it is used in our code.Here,I want all of you guys to share your knowledge of Regular expression.Thanks to all who shares their knowledge and their expertise here.

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Sathish Kumar.R
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Hi,
A regular expression is a string that is used to describe or match a set of strings, according to certain syntax rules. Regular expressions are used by many text editors, utilities, and programming languages to search and manipulate text based on patterns.

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Sathish Kumar.R
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A regular expression, often called a pattern, is an expression that describes a set of strings. They are usually used to give a concise description of a set, without having to list all elements. For example, the set containing the three strings "Handel", "Händel", and "Haendel" can be described by the pattern H(ä|ae?)ndel (or alternatively, it is said that the pattern matches each of the three strings). In most formalisms, if there is any regex that matches a particular set then there is an infinite number of such expressions.

Hi,
Do you know the namespace that is used to include the regular expression in c#.net?

Hi Deeban,
You must include the System.Text.RegularExpression classes inorder to perform the process of matching and replacing string with the help of regular expressions.

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Sathish Kumar.R
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Hi Sathish,
Can you explain the formalisms that is used in the Regular expression if you know?

Hi Deeban,
The most formalisms consists of
Alternation
A vertical bar separates alternatives. For example, gray|grey can match "gray" or "grey".
Grouping
Parentheses are used to define the scope and precedence of the operators (among other uses). For example, gray|grey and gr(a|e)y are equivalent patterns which both describe the set containing "gray" and "grey".
Quantification
A quantifier after a token (such as a character) or group specifies how often that preceding element is allowed to occur. The most common quantifiers are ?, *, and +.
? The question mark indicates there is zero or one of the preceding element. For example, colou?r matches both "color" and "colour".
* The asterisk indicates there are zero or more of the preceding element. For example, ab*c matches "ac", "abc", "abbc", "abbbc", and so on.
+ The plus sign indicates that there is one or more of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".

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Sathish Kumar.R
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Hi,
In the formal theory,Regular expressions can be expressed in terms of formal language theory. Regular expressions consist of constants and operators that denote sets of strings and operations over these sets, respectively. Given a finite alphabet Σ the following constants are defined:

(empty set) ∅ denoting the set ∅
(empty string) ε denoting the set {ε}
(literal character) a in Σ denoting the set {a}

Hi Deeban,
Also there are some operations in the regular expressions.They are
Concatenation:
(concatenation) RS denoting the set { αβ | α in R and β in S }. For example {"ab", "c"}{"d", "ef"} = {"abd", "abef", "cd", "cef"}.
Alteration:
(alternation) R|S denoting the set union of R and S.
Kleene Star:
(Kleene star) R* denoting the smallest superset of R that contains ε and is closed under string concatenation. This is the set of all strings that can be made by concatenating zero or more strings in R. For example, {"ab", "c"}* = {ε, "ab", "c", "abab", "abc", "cab", "cc", "ababab", ... }.

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Sathish Kumar.R
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Hi sathish,
Can you give some examples in the Regular expressions so that it is easy for me to understand?

Hi deeban,
Sure.I can give you the examples for the operations to you.Here are some examples
a|b* denotes {ε, a, b, bb, bbb, ...}
(a|b)* denotes the set of all strings consisting of any number of a and b symbols, including the empty string
ab*(c|ε) denotes the set of strings starting with a, then zero or more bs and finally optionally a c.

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Sathish Kumar.R
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Hi,
I couldnt find the System.Text.RegularExpressions namespace.So may I know the exact DLL used ?

Hi Deeban,
The regexp classes are contained in the System.Text.RegularExpressions.dll assembly, and you will have to reference this assembly .

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Sathish Kumar.R
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Hi Sathish,
Can we include the dll of the regular expression dynamically at compile time?

Hi Deeban,
Yes.We can include the dll at compile time by using the follwing code
csc /r:System.Text.RegularExpressions.dll foo.cs will build the foo.exe assembly, with a reference to the System.Text.RegularExpressions assembly.

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Sathish Kumar.R
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Hi sathish,
may I know what are the classed that are inside the System.Text.RegularExpression namespace?

Hi Deeban,
There are actually only six classes and one delegate definition in the assembly namespace. These are:

Capture: Contains the results of a single match

CaptureCollection: A sequence of Capture's

Group: The result of a single group capture, inherits from Capture

Match: The result of a single expression match, inherits from Group

MatchCollection: A sequence of Match's

MatchEvaluator: A delegate for use during replacement operations

Regex: An instance of a compiled regular expression

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Sathish Kumar.R
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Hi sathish,
In these classes,what class we mostly use for using Regular expressions?

Hi Deeban,
The Regex class is the class that we most often for the regular expressions.

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Sathish Kumar.R
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Hi sathish,
Can you tell me what are the methods that we use inside the Regex class?