Question

Can we use regular expressions to find out if a string follows these rules?

• Not have upper-case letters.
• Begin with a letter.
• Have at least 1 digit(s) not at the beginning and end.
• Have up to 8 alphanumeric
• Does NOT have any symbols like @#$ characters (symbols like !@#$%^&*()-+).

Answer 1

EDIT: I’ve updated the regular-expression in the code with one which looks to work.

@"^[a-z](?=[a-z0-9@#\$]*[0-9][a-z0-9@#\$]*)[a-z0-9@#\$]{0,6}[a-z@#\$]$"

EDIT: I’ve tweaked the code. One of the tests was aa1@aaaa marked as being expected to fail when it should succeed as pointed out by Douglas.

I’ve verified that the regular-expression offered by Douglas works just as well and its shorter too.

---

Not an answer as such, but here’s a quick test-rig. I’ve used the regular expression and tests from @ShaunWilde’s answer.

Unfortunately, the tests don’t return the expected results :-|.

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;

namespace StackOverflow.RegularExpressions
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var validString =
                new Regex(@"^[a-z][a-z\d!@#$%\^&*()\-+]{0,7}$(?<=\d\D+)",
                          RegexOptions.Compiled);

            var testsAndExpectedResults = new List<Tuple<string, bool>>
                                              {
                                                  new Tuple<string, bool>("a1@aaaaa", true),
                                                  new Tuple<string, bool>("a@1aaaaa", true),
                                                  new Tuple<string, bool>("aa@aaaaa", false),
                                                  new Tuple<string, bool>("a1@aaaaaaa", false),
                                                  new Tuple<string, bool>("a1@aaaa", true),
                                                  new Tuple<string, bool>("1a1@aaaa", false),
                                                  new Tuple<string, bool>("aa1@aaaa", true),
                                                  new Tuple<string, bool>("Aa1@aaaa", false),
                                                  new Tuple<string, bool>("Aa1@aaaA", false),
                                                  new Tuple<string, bool>("aA1@aaaA", false),
                                                  new Tuple<string, bool>("#A@a1aaaaa", false)
                                              };

            testsAndExpectedResults.ForEach(t =>
                                            Console.WriteLine("With '{0}' expected {1}, got {2}", t.Item1, t.Item2,
                                                              validString.IsMatch(t.Item1))
                );

            Console.ReadKey();
        }
    }
}