Regular Expression
- A Regular Expression is a set of characters that specify a pattern. It comes from a term used to describe grammars and formal language.
- All browsers that validate the email of the user use a Regular Expression.
- Regular Expressions are used when you want to search for specific lines of text containing a specific pattern.
- Regular Expressions search for patterns on a single line, and not for patterns that start on one line and end on another.
- Regular Expressions are more powerful and flexible. You can search for words of a certain size and Numbers, punctuation characters, you name it, a Regular Expression can find it.
Note
For checking we will use using System.Text.RegularExpressions;
Rules
- ^ from this our Regular Expression will start.
- $ At this sign our Regular Expression will end.
- + will concatenate the string.
- [a-zA-Z] This portion will restrict the user to enter always the first character as a letter because all standard emails must start with a letter.
- [a-zA-Z0-9] In this portion we can input alphabets and numbers.
- [[a-zA-Z0-9-_.!#$%'*+/=?^] In this portion we can input numbers, letters and special characters as described. All three of these portions will be concatenated with the + sign.
- {1,20} It tells the length of the email address.
- @[a-zA-Z0-9]{1,20} Here in this after @Regular Expression is a built in function that checks the domain pattern that we define. And also tells the pattern and length.
- .[a-zA-Z]{2,3}$ In this after it will check for .com or .pk or .in or .us.
Regular Expression
Diagram
Regex regex1 = new Regex("^[a-zA-Z]+[a-zA-Z0-9]+[[a-zA-Z0-9-_.!#$%'*+/=?^]{1,20}@[a-zA-Z0-9]{1,20}.[a-zA-Z]{2,3}$");
![Email checker]()
Program
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Text.RegularExpressions;
namespace CFGMailChecker
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
//Regular Expression
Regex regex1 = new Regex("^[a-zA-Z]+[a-zA-Z0-9]+[[a-zA-Z0-9-_.!#$%'*+/=?^]{1,20}@[a-zA-Z0-9]{1,20}.[a-zA-Z]{2,3}$");
//This function will check whether the expression is right or not.
if (!regex1.IsMatch(textBox1.Text))
{
MessageBox.Show("Your email address format is not correct!", "ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
else
{
MessageBox.Show("Congratulations Your email address format is correct!", "CORRECT", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
}
}
}
}
I have also attached the source code so you can download it and run in your PC.