How do I build a C# Regex to accurately validate music note formats?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I'm developing a C# project to handle music notes and need a regex that verifies the note input follows a specific format.

The requirements are:

Valid note letters: A, C, D, F, and G (which can optionally have a '#' appended) plus digits.
For notes B and E, the '#' is not allowed; they must be followed directly by one or more digits.

Valid examples:

C1
C#1
B1

Invalid examples:

B#1
CCCCC1
C####1
1C
I AM A FISH
C
H (or any character not in the defined list)
H#1
H1

Currently, I'm using the following code, but it incorrectly marks entries like "CCCCCCC1" and "B#1" as valid:

CSHARP
Regex h = new Regex(@"(((A|C|D|F|G)?(#?))|(B|E))?\d\d?", 
RegexOptions.IgnoreCase);
Match X = h.Match(Console.ReadLine());

if (X.Success)
{
    Console.WriteLine("VALID");
}
else
{
    Console.WriteLine("INVALID");
}
Console.ReadLine();

I'd appreciate any guidance on constructing a regex that properly accepts valid inputs (like "C1", "C#1", "B1") and rejects invalid ones.

Comments 0

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Answers 3

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3 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is one correct solution. When designing your regex, it’s best to anchor it (using ^ and $) so that the entire input is matched rather than a substring. In this case you want either:

• A note letter in the set A, C, D, F, or G (case insensitive) with an optional “#”

• Or a note letter in the set B or E (without a “#”)

After that, you require one or more digits.

A suitable regex is:

CSHARP
Regex h = new Regex(@"^(?:(?:[ACDFG]#?)|[BE])\d+$", RegexOptions.IgnoreCase);

Explanation

The caret (^) and dollar ($) anchors ensure that the entire string is matched.
The non-capturing group (?:(?:[ACDFG]#?)|[BE]) expresses that:
- Either one letter from A, C, D, F, or G optionally followed by a literal # (using #?), or
- One letter from B or E (with no sharp allowed)
Finally, \d+ ensures that one or more digits follow.
The RegexOptions.IgnoreCase flag handles case insensitivity.

This regex correctly handles your valid cases (e.g., “C1”, “C#1”, “B1”) and rejects invalid inputs like “B#1”, “CCCCCCC1”, etc.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by MartianSentinel125 • 1 month ago

Try This

CSHARP
 Regex h = new Regex(@"^([A-D|F-G]#?\d{1,2}|[B|E]\d{1,2})$",RegexOptions.IgnoreCase);

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Answer by CometWatcher629 • 1 month ago

I think this pattern can be simplified to just ^([ACDFG]#?|[BE])\d$

It matches all your valid inputs and rejects all your invalid. I have removed the capture groups for clarity, if you needed those just wrap the parentheses in the correct place ()

Your original code had a potential for 2 integers - you can use \d{1,2} to represent that if need be.

Demo: https://regexr.com/8b6vm

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How do I build a C# Regex to accurately validate music note formats?

3 Answers

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