How to check only 10 digit mobile number regex

Your regex works fine to check your sample data:

Regex reg = new Regex(@"^[0-9]{10}$");
string inputOK = "9876543214";
string inputBad1 = "876543214";
string inputBad2 = "09876543214";
if (reg.IsMatch(inputOK)) Console.WriteLine("OK");
if (!reg.IsMatch(inputBad1)) Console.WriteLine("OK");
if (!reg.IsMatch(inputBad2)) Console.WriteLine("OK");

It gives

OK
OK
OK

So ... the data you are checking is not what you think it is.
Use the debugger to check exactly what is in your input string when you validate it.

If you would like to check if mobile phone number is valid (10 digits), try this:

string[] mobilephones = {"9876543214", "987654321", "98765432142", "A876543214", "87C54d2142"};

//pattern
string pattern = @"^\d{10}$";

var result = mobilephones
	.Select(x=>new
	{
		mobile = x,
		IsValid = Regex.IsMatch(x, pattern),
	})
	.ToList();
	
foreach(var mb in result)
{
	Console.WriteLine("{0} - {1} valid mobile number", mb.mobile, mb.IsValid ? "is" : "is NOT");
}

Result:

9876543214 - is valid mobile number
987654321 - is NOT valid mobile number
98765432142 - is NOT valid mobile number
A876543214 - is NOT valid mobile number
87C54d2142 - is NOT valid mobile number

For further details, please see: Regex.IsMatch Method[^]

Regex(@"^[0-9]{10}$");

use this it will work properly. it will also check if any alphabets are there or not. in this we can only enter 10 digits number

val UserMobile = findViewById<edittext>(R.id.UserMobile)
val msgUserMobile: String = UserMobile.text.toString()

fun String.isMobileValid(): Boolean {
// 11 digit number start with 011 or 010 or 015 or 012
// then [0-9]{8} any numbers from 0 to 9 with length 8 numbers
if(Pattern.matches("(011|012|010|015)[0-9]{8}", msgUserMobile)) {
return true

}
return false
}
if(msgUserMobile.trim().length==11&& msgUserMobile.isMobileValid())
{//pass} else {//not valid}

a = int(input("please enter the number: "))
if a.isdigits() and (a.startswith('7') or a.startswith('8') or a.startswith('9')) and len(a) == 10:
print('YES')
else:
print('NO')