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