Using Regex to recognize pattern in string

Question

I apologize if this is a repeat question, as I know there are many about Regex on StackOverflow, but I have yet to find an answer or a level of help I need.

I have a string that needs to be a length of 8 where:

The first two characters are letters

The next five characters are numbers

The last character is a letter

For example: "AB12345C"

I have been using the examples from MSDN & DotNetPerls to try and understand how to use arguments properly, but after a couple days of reading around I still can't get it to pass.

I am currently trying to use:

public Tuple<bool, string> stringFormatCheck(string input)
{            
     if (Regex.IsMatch(input, @"^[a-zA-Z]\d{2}[0-9]\d{5}[a-zA-Z]$") == true)
        return Tuple.Create(true, "String is Fine");
     else
        return Tuple.Create(false, "String Format is incorrect");
}

Can someone show me how to use this argument properly or somewhere I can get a better understanding of the Regex Class? Thank you.

EDIT1: The second Z in my first argument is now capitalized.

Tools like "expresso" (ultrapico.com/expresso.htm) or "the regulator" will help you analyze, build and test your regular expression, have you tried one of those? — Paolo Tedesco
– Paolo Tedesco, Commented Apr 21, 2016 at 15:30
@PaoloTedesco No, I had no idea these tools existed. I will definitely look into those, thank you! — Rinktacular
– Rinktacular, Commented Apr 21, 2016 at 15:30
Your first [a-zA-z] is wrong. Second Z should be uppercase. Curious how you're testing your regex and what convinces you it's not working. — adv12
– adv12, Commented Apr 21, 2016 at 15:31
@adv12 From my understanding, Regex.IsMatch will return true if the input string matches the Regex arguments. When I pass in a string that I assign myself, which I know fit the pattern I am trying to match, it has only returned false this far. — Rinktacular
– Rinktacular, Commented Apr 21, 2016 at 15:33
@Rinktacular: put your regex in Expresso -> regular expression panel: the "regex analyzer" panel will show you immediately what's wrong :) — Paolo Tedesco
– Paolo Tedesco, Commented Apr 21, 2016 at 15:35

Dmitrii Bychenko · Accepted Answer · 2016-04-21 15:44:03Z

1

The right pattern is

"^[A-Za-z]{2}[0-9]{5}[A-Za-z]$"

with, IMHO, clear interpretation:

^           - string start (anchor)
[A-Za-z]{2} - 2 letters A..Z or a..z
[0-9]{5}    - 5 digits 0..9
[A-Za-z]    - letter A..Z or a..z
$           - string end (anchor)

And so the implementation can be

public Tuple<bool, string> stringFormatCheck(string input)
{            
    if (Regex.IsMatch(input, @"^[A-Za-z]{2}[0-9]{5}[A-Za-z]$"))
        return Tuple.Create(true, "String is Fine");
    else
        return Tuple.Create(false, "String Format is incorrect");
}

Please, notice, that [0-9] is a better choice than \d since you, probably, don't want let, say, Persian digits like "AB۰۱۲۳۴C" ;

edited Apr 21, 2016 at 15:44

answered Apr 21, 2016 at 15:37

Dmitrii Bychenko

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5 Comments

Rinktacular Over a year ago

Thank you very much! Just out of curiosity, are the capital A-Z required to go before the lower case a-z in the first and last arguments?

Dmitrii Bychenko Over a year ago

@Rinktacular: no, the order can be arbitrary A-Za-z as good as a-zA-Z

Rinktacular Over a year ago

Fantastic, the code passes as expected now. Thank you very much, I can see where I was wrong now and can properly use this in my future projects!

Dmitrii Bychenko Over a year ago

You're welcome! In your current pattern, just drop both \d

Rinktacular Over a year ago

I kept seeing /d in almost every example I saw online so I was led to believe that was important to the argument I was making ha. Your example is very clear and very helpful, thanks again!

adv12 · Accepted Answer · 2016-04-21 15:41:18Z

1

Try this: ^[a-zA-Z]{2}[0-9]{5}[a-zA-Z]$

Your regex: ^[a-zA-z]\d{2}[0-9]\d{5}[a-zA-Z]$ doesn't work for multiple reasons. First, the second z should be capitalized. Then, the first \d is trying to match a digit, so you're saying "Match any letter then two digits." You make the same mistake with the second \d: you say "Match any digit ([0-9]) and then match 5 digits (\d{5}).

edited Apr 21, 2016 at 15:41

answered Apr 21, 2016 at 15:39

adv12

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4 Comments

Dmitrii Bychenko Over a year ago

(Possible) counter example: "AB۰۱۲۳۴C" is accepted, when I'm sure it should be rejected; please notice Persian digits.

Wiktor Stribiżew Over a year ago

@DmitryBychenko: That can be helped with a RegexOptions.ECMAScript.

adv12 Over a year ago

@DmitryBychenko, thanks. I've replaced the \d with [0-9].

Rinktacular Over a year ago

Thank you, I got my answer from the other one on this question, but both of you have given me a much better understanding of Regex arguments for my use of them.

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