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• Your solution looks fine;
• Overall, we would want to stay away from regular expressions in solving algorithm problems – especially easy LeetCode questions are to measure your implementation skills;
• C# has two simple functions that would do the job of that regular expression:

• We can just a bit improve the runtime, but not so much, using a more readable expression (?i)[^a-z0-9]+:

• Your solution looks fine;
• Overall, we would want to stay away from regular expressions in solving algorithm problems – especially easy LeetCode questions are to measure our implementation skills;
• C# has two simple functions that would do the job of (\W|_):

• We can just a bit improve the runtime, but not so much, using a more readable expression such as:

  • (?i)[^a-z0-9]+;

    • (?i) matches the remainder of the pattern based on the insensitive flag;

  • [^A-Za-z0-9]+;

Demo

Disclaimer: Not a Code Reviewer

Disclaimer: Not a Code Reviewer

Happy Coding! ( •ˆ_ˆ• )

• We can just a bit improve the runtime, but not so much:

public class Solution {
 public bool IsPalindrome(string s) {
 s = System.Text.RegularExpressions.Regex.Replace(s, @"(?i)[^a-z0-9]", "");
 if (s == null || s == "" || s.Length == 1 || s == " ") {
 return true;
 }
 for (int i = 0, j = s.Length - 1; i <= s.Length / 2 && j >= s.Length / 2; i++, j--) {
 if (Char.ToLower(s[i]) == Char.ToLower(s[j])) {
 continue;
 }
 return false;
 }
 return true;
 }
}```

• We can just a bit improve the runtime, but not so much, using a more readable expression (?i)[^a-z0-9]+:

public class Solution {
 public bool IsPalindrome(string s) {
 s = System.Text.RegularExpressions.Regex.Replace(s, @"(?i)[^a-z0-9]+", "");
 if (s == null || s == "" || s.Length == 1 || s == " ") {
 return true;
 }
 for (int i = 0, j = s.Length - 1; i <= s.Length / 2 && j >= s.Length / 2; i++, j--) {
 if (Char.ToLower(s[i]) == Char.ToLower(s[j])) {
 continue;
 }
 return false;
 }
 return true;
 }
}

RegEx Circuit

jex.im visualizes regular expressions:

enter image description here

If you wish to simplify/update/explore the expression, it's been explained on the top right panel of regex101.com . You can watch the matching steps or modify them in this debugger link , if you'd be interested. The debugger demonstrates that how a RegEx engine might step by step consume some sample input strings and would perform the matching process.