Need help to build regular expression to match a specific string (example inside)

Question

Hi,

I'm working on a program where I need to match a regular expression and a string. The string is pretty simple actually but I'm having problems with my current regex (I'm using the .net regex engine)

My current regular expression is "^[VvEeTtPp][^a-zA-Z0-9\s]\d{0,12}?$"

Now, the string I want to match always follows this pattern

1. First a single letter (only letters allowed are V, E, P, T in either case)
2. Then, a dash
3. Finally from 4 to 12 digits.

There is a final restriction that is that the regex must match any substring that complies the rules (for example "V" or "E-" or "P-123")

The regex works fairly well, but it will accept things like "V--".

Could someone help me write a better expression??

Thanks

Answer 1

Well, a substring of the 4-12 rule really just makes it a 1-12 rule, so how about:

        Regex re = new Regex(@"^[VvEeTtPp](-|-[0-9]{1,12})?$");
        Console.WriteLine(re.IsMatch("B"));
        Console.WriteLine(re.IsMatch("V"));
        Console.WriteLine(re.IsMatch("E-"));
        Console.WriteLine(re.IsMatch("P-123"));
        Console.WriteLine(re.IsMatch("V--"));

Answer 2

This should do it:

^[EPTVeptv](-(\d{4,12})?)?$

Edit:
To also match substrings like "P-123", "-123" and "123":

^(?=.)[EPTVeptv]?(-\d{,12})?$

Edit 2:
Added a positive lookahead in the beginning, so that the pattern doesn't match the substring "". Although that is a valid substring of a legal value, I assume that you don't want that specific substring...

Answer 3

[VvEePpTt]-\d{4,12}

Answer 4

Can you try this and tell me if it works?

^[VvEeTtPp](-(\d{4,12}){0,1}){0,1}$

It will accept a single character of the ones specified followed by either nothing or one dash which in turn is either not followed by 4-12 digits or 4-12 digits and matches them. For instance :

• V
• V-12
• V12
• V-12345
• P-1234567890123

EDIT : Added a $ at the end so it will fail if the string contains any extra characters

Answer 5

I think this pattern fits the specification.

string pattern = @"^[VvEePpTt](?:$|-(?:$|\d{1,12}$))";
// these are matches
Console.WriteLine(Regex.IsMatch("V", pattern));
Console.WriteLine(Regex.IsMatch("v-", pattern));
Console.WriteLine(Regex.IsMatch("P-123", pattern));
Console.WriteLine(Regex.IsMatch("t-012345678901", pattern));
// these are not
Console.WriteLine(Regex.IsMatch("t--", pattern));
Console.WriteLine(Regex.IsMatch("E-0123456789012", pattern));

Pattern breakdown:

^             - start of string
[VvEePpTt]    - any of the given characters, exactly once
(?:           - start a non-capturing group...
$|-           - ...that matches either the end of the string or exactly one hyphen
(?:           - start a new non-capturing group...
$|\d{1,12}$   - that matches either the end of the string or 1 to 12 decimal digits
))            - end the groups