JavaScript extra possible character in regex expression

Question

I'm making a program about music chords. The first character has to be a single letter A through G. The second letter can contain an optional either "#", "b" or "" (nothing at all). And then I want a capital M followed by a 7.

For example, in one of my boolean statements I want "AM7" and "F#M7" to both evaluate to true, for both being a major chord in music theory.

Would this be something like:

/([A-G]{#b''}{M7})/$.test("GbM7");    //Should be true
/([A-G]{#b''}{M7})/$.test("GbbM7");    //Should be false
/([A-G]{#b''}{M7})/$.test("Gbm7");    //Should be false

I know the code above is wrong, but I am trying to illustrate my throught process of what i am going for. I feel like I am really close.

Thanks for any help!

Well the regex part for "either '#', 'b', or nothing" would be [b#]? — Pointy
– Pointy, Commented Jul 12, 2017 at 20:21
I'm dumb. I thought the curly braces were used when there was a set set value that it could take on, as opposed to a range. I also thought that the money sign was used to end a regex. Sorry, regex is really hard for me to grasp for some reason. — Team. Coco
– Team. Coco, Commented Jul 12, 2017 at 20:28

Barmar · Accepted Answer · 2017-07-12 20:21:38Z

3

Use ? to mark something optional.

/[A-G][#b]?M7/

answered Jul 12, 2017 at 20:21

Barmar

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13 Comments

buttonupbub Over a year ago

You could use a quantifier in that case: /([ABCDE][#b]?){1,5}M7/

Barmar Over a year ago

@Team.Coco [A-G]{1,5} will match from 1 to 5 occurrences of that pattern.

buttonupbub Over a year ago

For completeness, you might want to use beginning and end markers to rule out partial matches: /^([A-G][#b]?){1,5}M7$/. Otherwise, a string like 'dinosaurA#M7' would pass.

Barmar Over a year ago

@buttonupbub It depends on whether this is for validation of a whole string or searching text for a match.

Barmar Over a year ago

{1} is the default for any pattern, it matches 1 time unless there's a quantifier that modifies that.

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buttonupbub · Accepted Answer · 2017-07-12 20:35:50Z

2

I believe the expression you are looking for is

/[A-G][#b]?M7/

The ? marks the previous element ([#b]) as optional.

edited Jul 12, 2017 at 20:35

answered Jul 12, 2017 at 20:22

buttonupbub

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2 Comments

Team. Coco Over a year ago

Thank you for your answer. I chose the above as best answer since they posted just a few seconds before you, but I appreciate your response!

buttonupbub Over a year ago

Thanks for the feedback. Just joined Stack Overflow. Wow, these answers come out fast.

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