if(expression == null or expression.Length < 36) return false;

return Regex.IsMatch(expression, ...);

Shorter version:

^{?([0-9a-fA-F]){8}(-([0-9a-fA-F]){4}){3}-([0-9a-fA-F]){12}}?$

BUT, both our versions match:

things like:

{CA761232-ED42-11CE-BACD-00AA0057B223

which is wrong

This is just asking to go wrong and spoil someone's day six months down the line. Just try to instantiate a new Guid and catch FormatException.

Hasn't gone wrong for me for 2 years

You could do this, which is at least correct:

        public static bool IsGuid(string text)
        {
                return
                (
                        new Regex
                        (@"
                                (
                                        \A{
                                                [a-z\d]{8}
                                                (-[a-z\d]{4}){3}
                                                -[a-z\d]{12}
                                        }\z
                                )
                                |
                                (
                                        \A
                                                [a-z\d]{8}
                                                (-[a-z\d]{4}){3}
                                                -[a-z\d]{12}
                                        \z
                                )
                        ",
                                        RegexOptions.IgnoreCase
                                |        RegexOptions.IgnorePatternWhitespace
                                |        RegexOptions.CultureInvariant


                        )
                ).IsMatch(text);
        }

Or you could do as suggested previously and catch FormatException, which I'd consider equally valid.

Let's see how well this site mangles my code, shall we?

Thanks Rik H, removed the mangling

What's wrong with Guid.TryParse();

Rik Hemsley: [a-z] is wrong. You only want to match hex digits.

The problem with Guid.TryParse is that it does not exist!

I think a TryParse method would be more consistent with the FrameWork. (And with extension methods it could also be added to the Guid class - in the not so distant future)

For this reason I prefer this Implementation

http://geekswithblogs.net/colinbo/archive/2006/01/18/66307.aspx

And Here is a link with a feature request for Microsoft to implement it:

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=94072

and here is the link with feedback on the issue with proposed workaround and some tests.

http://connect.microsoft.com/VisualStudio/feedback/Workaround.aspx?FeedbackID=94072

Trapping the error would be slow that's the purpose Paul has created this function in the first place!

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=94072

Curiously though, TryParse DOES exist on the Guid class, it's just private. You'd think they could have just made it public...

"Just try to instantiate a new Guid and catch FormatException."

This works, but it's inelegant and can be slow. IMHO, validating with a reg ex is more elegant and should run faster than to catch the exception because you're too lazy to validate. I'm just one of those guys who feels that throwing an exception after a bad user entry (implicit validation) is not as good as a reg ex (explicit validation).

this one catches all kinds of guids in following formats

('d' represents a hexadecimal digit whose case is ignored): 32 contiguous digits: dddddddddddddddddddddddddddddddd or- Groups of 8, 4, 4, 4, and 12 digits with hyphens between the groups. The entire GUID can optionally be enclosed in matching braces or parentheses: dddddddd-dddd-dddd-dddd-dddddddddddd -or- {dddddddd-dddd-dddd-dddd-dddddddddddd} -or- (dddddddd-dddd-dddd-dddd-dddddddddddd) -or Groups of 8, 4, and 4 digits, and a subset of eight groups of 2 digits, with each group prefixed by "0x" or "0X", and separated by commas. The entire GUID, as well as the subset, is enclosed in matching braces: {0xdddddddd, 0xdddd, 0xdddd,{0xdd,0xdd,0xdd,0xdd,0xdd,0xdd,0xdd,0xdd}}

Regular Expression:

        static readonly Regex regGuid =

            new Regex

                (@"

                    ^(

                        (

                            \A\(

                                [a-f\d]{8}

                                (-[a-f\d]{4}){3}

                                -[a-f\d]{12}

                            \)\z

                        )

                        |

                        (

                            \A\{

                                [a-f\d]{8}

                                (-[a-f\d]{4}){3}

                                -[a-f\d]{12}

                            \}\z

                        )

                        |

                        (

                            \A\{\s*

                                0x[a-f\d]{8}\s*,\s*

                                (0x[a-f\d]{4}\s*,\s*){2}

                                \{\s*

                                    (0x[a-f\d]{2}\s*,\s*){7}

                                    0x[a-f\d]{2}\s*

                                \}\s*

                            }\z

                        )

                        |

                        (

                            \A

                                [a-f\d]{32}

                            \z

                        )

                    )$

                ",

                  RegexOptions.Compiled | 

                  RegexOptions.IgnorePatternWhitespace | 

                  RegexOptions.IgnoreCase

              );

Using:

        public static bool IsGuid(string g)

        {

            return !string.IsNullOrEmpty(g) || regGuid.IsMatch(g);

        }

Ok i looked into this and decided a better method is to use an extention method. (Server Side Validation) so for anyone who needs to validate on the server and hates RegEx here you go.

http://www.cochrane.ws/post/VBNET-IsGuid-Extension-Method.aspx

this fixes tclark's problem of mis-matched braces:

^?([0-9a-fA-F]){8}(-([0-9a-fA-F]){4}){3}-([0-9a-fA-F]){12}(?(1)})$

Thanks for the assist; here is a vb version....

    Public Function isGUID(ByVal QFormUID As String) As Boolean

        Dim guidRegEx As Regex = New Regex("^(\{{0,1}([0-9a-fA-F]){8}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){12}\}{0,1})$")

        If guidRegEx.IsMatch(QFormUID) Then

            Return True

        Else

            Return False

        End If

    End Function

Here is a version that is a little more "tight":

        public static bool IsGuid(string expression)

        {

            bool isGuid = false;

            if (!string.IsNullOrEmpty(expression))

            {

                Regex guidRegEx = new Regex(@"^(\{{0,1}([0-9a-fA-F]){8}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){12}\}{0,1})$");

                isGuid = guidRegEx.IsMatch(expression);

            }

            return isGuid;

        }

Why not /\w{8}-(\w{4}-){3}\w{12}/