Using C# Regex.IsMatch to validate 4 digit string?

[Byron]

All I am trying to do is use Regex.IsMatch to validate that a string is
either empty, or exactly 4 characters (digits) in length. In other words,
the strings, "", and "1234" should pass, whereas "123", "abcd", and "12345"
should fail. I know there has to be a simple answer to this, but right now
I can't find it. I tried using:

if (str.Length == 0)

return true;

if (Regex.IsMatch( str, "[0-9]^4"))

return true;

but this does not work. Variations such as

Regex.IsMatch(" 1234", @"\d\d\d\d") come close, but also result in "12345"
validating.

Any help would be greatly appreciated.

[Ignacio Machin \( .NET/ C# MVP \)]

Hi Byron,

You have to use ^ and $ to indicate start and end of the string:
Regex.IsMatch(" 1234", @"^\d\d\d\d$ ")

should do the trick.

Cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"Byron" <si*****@ctcorp .com> wrote in message
news:e0******** ******@TK2MSFTN GP10.phx.gbl...

All I am trying to do is use Regex.IsMatch to validate that a string is
either empty, or exactly 4 characters (digits) in length. In other words,
the strings, "", and "1234" should pass, whereas "123", "abcd", and "12345" should fail. I know there has to be a simple answer to this, but right now I can't find it. I tried using:

if (str.Length == 0)

return true;

if (Regex.IsMatch( str, "[0-9]^4"))

return true;

but this does not work. Variations such as

Regex.IsMatch(" 1234", @"\d\d\d\d") come close, but also result in "12345"
validating.

Any help would be greatly appreciated.

[Byron]

That did the trick. Thanks for the quick reply
"Ignacio Machin ( .NET/ C# MVP )" <ignacio.mach in AT dot.state.fl.us > wrote
in message news:OF******** *****@TK2MSFTNG P12.phx.gbl...

Hi Byron,

You have to use ^ and $ to indicate start and end of the string:
Regex.IsMatch(" 1234", @"^\d\d\d\d$ ")

should do the trick.

Cheers,

--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

"Byron" <si*****@ctcorp .com> wrote in message
news:e0******** ******@TK2MSFTN GP10.phx.gbl...

All I am trying to do is use Regex.IsMatch to validate that a string is
either empty, or exactly 4 characters (digits) in length. In other words, the strings, "", and "1234" should pass, whereas "123", "abcd", and

"12345"

should fail. I know there has to be a simple answer to this, but right

now

I can't find it. I tried using:

if (str.Length == 0)

return true;

if (Regex.IsMatch( str, "[0-9]^4"))

return true;

but this does not work. Variations such as

Regex.IsMatch(" 1234", @"\d\d\d\d") come close, but also result in "12345" validating.

Any help would be greatly appreciated.