Question

I need to verify that a string is in a certain format…here are the rules.

• Can contain a colon and/or dot.
• Both the colon and dot are optional
• If a colon and/or dot is specified there must be at least one character to the left and one character to the right of the colon/dot.
• The colon must be before the dot if both are specified
• Only 0 or 1 colon and 0 or 1 dot is allowed
• AnyString means a string of one or more unicode characters excluding colon and dot (colon and dot characters are not allowed as part of AnyString).

Examples:

Can be…

AnyString:AnyString.AnyString
AnyString:AnyString
AnyString.AnyString
AnyString

Cannot be…

AnyString:.AnyString
AnyString.AnyString:AnyString
AnyString:
AnyString.
:AnyString
.AnyString

I have tried lots of different combinations and I am just not good enough at Regular Expressions to get this one.

Thanks in advance

Answer 1

Well, that looks like:

• It definitely starts with one or more non-colon-or-dot characters
• It then optionally has a colon followed by one or more non-colon-or-dot characters
• It then optionally has a dot followed by one or more non-colon-or-dot characters
• If there are both “colon plus X” and “dot plus X” sections, the colon section must come first

(Note that none of your now-edited-in explanation was present when I wrote the above, so it was just based on the examples.)

So I’d expect that to be a regex like this:

^[^.:]+(?::[^.:]+)?(?:\.[^.:]+)?$

Notes:

• You’d want to put all of this in a verbatim string literal to avoid having to escape the backslashes, e.g.

  var regex = new Regex(@"^[^.:]+(?::[^.:]+)?(?:\.[^.:]+)?$");
  

• ^ matches the start of a string

• [^.:] will match any character other than dot or colon
• + is the syntax for “at least one”
• (?:<subexpression>) is the syntax for a non-capturing group
• \. is an escaped dot, as . means “any character”
• ? is the syntax for “zero or one” (i.e. optional)
• $ matches the end of a string

Test code:

using System;
using System.Text.RegularExpressions;

class Test
{
    static readonly Regex regex =
        new Regex(@"^[^.:]+(?::[^.:]+)?(?:\.[^.:]+)?$");

    static void Main()
    {
        AssertValid("AnyString:AnyString.AnyString",
                    "AnyString:AnyString",
                    "AnyString.AnyString",
                    "AnyString");

        AssertInvalid("AnyString:.AnyString",
                      "AnyString.AnyString:AnyString",
                      "AnyString:",
                      "AnyString:..Anystring",
                      "AnyString.",
                      ":AnyString",
                      ".AnyString");
    }

    static void AssertValid(params string[] inputs)
    {
        foreach (var input in inputs)
        {
            if (!regex.IsMatch(input))
            {
                Console.WriteLine("Expected to match but didn't: {0}",
                                  input);
            }
        }
    }

    static void AssertInvalid(params string[] inputs)
    {
        foreach (var input in inputs)
        {
            if (regex.IsMatch(input))
            {
                Console.WriteLine("Expected not to match but did: {0}",
                                  input);
            }
        }
    }
}