Is valid identifier?

Given a string, determine if it's a valid identifier.

Here is the syntax for valid identifiers:

• Each identifier must have at least one character.
• The first character must be picked from: alpha, underscore, or dollar sign. The first character can not be a digit.
• The rest of the characters (besides the first) can be from: alpha, digit, underscore, or dollar sign. In other words, it can be any valid identifier character.

Examples of valid identifiers:

• i
• wo_rd
• b2h

Examples of invalid identifiers:

• 1i
• wo rd
• !b2h

using System;
using System.Text.RegularExpressions;
public class IdentifierChecker
{
  public static bool IsValid(string input)
        {
            string pattern = @"^[A-Za-z_$][\w_$]+$";
            Regex regex = new Regex(pattern);
            return regex.IsMatch(input);
        }
}

^...$开头和结尾

[A-Za-z_$] 第一个字符的可选范围
[\w_$]后续字符的可选范围  
+ 一个或多个重复

One or more repetitions