Question

in c#

Regular Expressions. What are they? What do they do for us? What is their power and what challenges to they present to writing solid code?

Answer 1

Regular Expressions

A regular expression, regex or regexp (sometimes called a rational expression) is a sequence of characters that define a search pattern. Usually such patterns are used by string searching algorithms for "find" or "find and replace" operations on strings, or for input validation. It is a technique developed in theoretical computer science and formal language theory.

Benefits of using Regular Expression

• Wide range of usage possibility, you may create one regular expression to validate any kind of input;
• Supported by almost any language, there are only a few programming languages which do not understand regular expressions;
• Do more with less, keep your code cleaner;
• Faster validations, instead of having many IF and ELSE operators you may validate only once with a regular expression.

Regular expressions are used in search engines, search and replace dialogs of word processors and text editors, in text processing utilities such as sed and AWK and in lexical analysis. Many programming languages provide regex capabilities either built-in or via libraries.

For example: In C#, Regular Expression is a pattern which is used to parse and check whether the given input text is matching with the given pattern or not. In C#, Regular Expressions are generally termed as C# Regex.

C# provides a class termed as Regex which can be found in System.Text.RegularExpression namespace. This class will perform two things:

• Parsing the inputing text for the regular expression pattern.
• Identify the regular expression pattern in the given text.

Below example demonstrate the use of regex in Mobile Number Verification. Suppose you are making a form where you need to verify the user-entered mobile number then you can use regex.

// C# program to validate the Mobile

// Number using Regular Expressions

using System;

using System.Text.RegularExpressions;

class GFG {

          

                // Main Method

                static void Main(string[] args)

                {

                                // Input strings to Match

                                // valid mobile number

                                string[] str = {"9925612824",

                                "8238783138", "02812451830"};

                               

                                foreach(string s in str)

                                {

                                                Console.WriteLine("{0} {1} a valid mobile number.", s,

                                                                                                isValidMobileNumber(s) ? "is" : "is not");

                                }

                               

                                Console.ReadKey();

                }

               

                // method containing the regex

                public static bool isValidMobileNumber(string inputMobileNumber)

                {

                                string strRegex = @"(^[0-9]{10}$)|(^\+[0-9]{2}\s+[0-9]

                                                                {2}[0-9]{8}$)|(^[0-9]{3}-[0-9]{4}-[0-9]{4}$)";

                               

                                // Class Regex Repesents an

                                // immutable regular expression.

                                // Format                                            Pattern

                                // xxxxxxxxxx                     ^[0 - 9]{ 10}$

                                // +xx xx xxxxxxxx            ^\+[0 - 9]{ 2}\s +[0 - 9]{ 2}\s +[0 - 9]{ 8}$

                                // xxx - xxxx - xxxx ^[0 - 9]{ 3} -[0 - 9]{ 4}-[0 - 9]{ 4}$

                                Regex re = new Regex(strRegex);

                               

                                // The IsMatch method is used to validate

                                // a string or to ensure that a string

                                // conforms to a particular pattern.

                                if (re.IsMatch(inputMobileNumber))

                                                return (true);

                                else

                                                return (false);

                }

}

Output:

9925612824 is a valid mobile number.

8238783138 is a valid mobile number.

02812451830 is not a valid mobile number.

There are many basic syntaxes like Quantifiers, Special Characters, Character Classes, Grouping & Alternatives are used for regular expressions.

Using regular expressions in any language seems to be challenging to provide the exact things what user says....