Learn Regular Expression in 15-Minutes

Learn Regular Expression in 15-Minutes

  01:34    Sarvesh Kushwaha

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What are Regular Expressions and Why we should use it ?
CodeProject
Regular expressions are used to search, replace ,split  ,match  for substrings ("matches") in strings. This is done by searching with "patterns" through the string .

Where To use Regular Expressions ?


Regular Expressions are one of the foundations of the Perl programming language and therefore built-into the compiler itself. Many languages have In-built Regular Expression or they support it via Add-ons.
Microsoft's .NET framework (including C#) , PHP , JAVA, VBScript , JScript , JavaScript ,Typescript, Jquery are languages and libraries which supports regular expressions.

In asp.net you will have to add a namespace to use REGEX .

using System.Text.RegularExpressions;

A very Simple Program to demonstrate REGEX.

using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Regex RE = new Regex("[A-Za-z]{6}");

            Console.WriteLine(RE.IsMatch("Sarvesh"));

            Console.ReadLine();  // it will hold your screen untill it gets any input
        }
    }
}

Above Program will match your string with Upper and lower alphabets and string should be of length Six .output will be True.

Rules and Things To Remember For Regular Expressions -

you can divide regular expression in three parts -

The following table summarizes the Compulsory syntax for regular expressions:

Sign	Description
^	It specifies START of Regular Expression.
$	It specifies the END of Regular Expression.
[]	Square brackets specify the character which needs to be matched.  Examples : [A-Za-z] means it will only accept Upper and lower   alphabets. [0-9] will accept only numeric numbers .
{}	Curly brackets specify how many characters.  Examples : {10} means length of string should be 10 digits.{1,4} means {min , max} length of string will be in b/w or equal to min and  max.
()	Round brackets specify grouping.

Example :
now lets test what we have learned .create a regular expression for indian mobile number validation .To create mobile number validation what we know , that mobile number should be of 10 numeric digits.so regular expression will be -

^[0-9]{10}$

above expression tells that string should be only numeric i.e [0-9].and {10} fixed digits.but still this expression can take 0123456789 .that is not a valid mobile number , because mobile number can't start from zero.so lets enhance this regular expression.

^[1-9]{1}[0-9]{9}$

now first digit should be in b/w 1 to 9 .now make this expression more perfect group your digits with round brackets : ^([1-9]{1})([0-9]{9})$

Shortcuts and Quantifiers :

Shorcuts/Quantifier	Description
*	Zero or more matches.
.	Matches any character except \n
+	One or more matches.
?	Zero or one matches.
\d	Matches any decimal character
\w	Matches any alphanumeric character and underscore.

to learn in depth about  REGEX please go to this link : Cheat Sheet for Refular Expression,

Regular expression in Depth.