.NET regular expression for a relatively simple string format [closed]

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I'm trying to construct a regular expression for the following:

• 6 characters exactly
• Alphanumerical characters only
• Letters are all uppercase
• There are exactly 1, 3, 4 or 5 letters at the beginning, the rest must be numbers

How do I go about that?

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Here's how I'd write it (using C# in free-spacing mode):

if (Regex.IsMatch(text, @"
    # Match specific string with multiple requirements.
    ^                      # Anchor to start of string.
    (?=.{6}$)              # 6 characters exactly.
    (?:                    # There are exactly
      [A-Z]{3,5}           # 3, 4, 5 or
    | [A-Z]                # 1, uppercase letters
    )                      # at the beginning,
    [0-9]+                 # the rest must be numbers.
    $                      # Anchor to end of string.", 
    RegexOptions.IgnorePatternWhitespace)) {
    // Successful match
} else {
    // Match attempt failed
} 

Edit: Changed from PHP to C#/.NET syntax.

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I'd prefer to use more than just regex to get more humaa readability

var input = "ABC456";
return input.Length==6 && Regex.IsMatch(input,@"^([A-Z]|[A-Z]{3,5})\d+$");

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Abdicate

Definition refactor:
- 6 total characters, capital letter and number chars only
- first must be a letter, last a number - first letter is optionally followed by 2 to 4 letters

Method 1:

^(?=.{6}$)[A-Z](?:[A-Z]{2,4})?\d+$

(expanded)

^
(?= .{6} $ )
[A-Z] (?:[A-Z]{2,4})? \d+ $ 

Method 2:

^(?=[A-Z](?:(?<!\d)[A-Z]|\d){4}\d$)[A-Z](?:[A-Z]{2,4})?\d

(expanded)

^
(?= [A-Z] (?: (?<!\d)[A-Z] | \d ){4} \d $ )
[A-Z] (?:[A-Z]{2,4})? \d

Method 2 doesn't require an end anchor because the assertion is length and end specific, but is made to appear slow/cumbersome by the lookbehind (due to OP's conditions).

For this reason I would go with a Method 1 regex, although I believe the other would obtain better speed bench's, which is probably irrelavent over code clarity in this case.

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^[A-Z](([A-Z]{2}[0-9]{3})|([A-Z]{3}[0-9]{2})|([A-Z]{4}[0-9]{1})|([0-9]{5}))$

UPDATED: Now matches all requirements

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^([A-Z]\d{5})|([A-Z]{3}\d{3})|([A-Z]{4}\d{2})|([A-Z]{5}\d)$

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This should do the trick on the top of my head. There may be shorter more elegant versions..

bool foundMatch = false;
try {
    foundMatch = Regex.IsMatch(subjectString, @"\A(?:(?:[A-Z]\d{5})|(?:[A-Z]{3}\d{3})|(?:[A-Z]{4}\d{2})|(?:[A-Z]{5}\d))\Z");
} catch (ArgumentException ex) {
    // Syntax error in the regular expression
}