How can I reject a string containing non-consecutive characters in a date format

Question

I'm trying to come up with a regex which will reject non-consecutive characters in a supplied date format. I want to be as flexible as possible, and so I have decided that my date format string can contain YY or YYYY, MM or MMM, DD or DDD, hh mm and ss.

Some of the regex I have worked out already - for example, matching the following will show that the month is a 3 character format:

([M])\\1{2}

I'm totally in the dark with regard to checking that the date format doesn't contain non consecutive characters. For example, the following date formats should be valid:

YYYY-MM-DD hh:mm:ss
hh:mm:ss YYYY-MM-DD 
DD/MMM/YYYYhh-mm

But these formats should be rejected

YYYY-MM-DD hh:mm:ss YYYY // year appears twice
hh:mm:ss YYYY-MM-DD hh // hour appears twice
DD/MMM/YYYYhh-mm m // m not consecutive with other m

In the interests of future expansion, I want to allow non consecutive special characters (/ - . : ) etc and reject all non-consecutive alpha-numeric characters. Case sensitive though - mm and MM are not not the same (as above)

Just to be clear - I'm not trying to validate an actual date - I am trying to validate a date format string only.

Answer 1

I suggest checking if there is at least one occurrence of the same character that has already been present before, and then negating the outcome:

function(text) { 
    return !/(\w)\1*(?!\1).*\1/.test(text);
}

See the regex demo . You may change \\w to [YMDhms] to only check these six letters.

Pattern details

• (\\w) - Group 1 (further referenced to with the \\1 backreference): a word char
• \\1* - zero or more occurrences of the same char as in Group 1
• (?!\\1) - set a boundary, make sure there next char is not the same as the char in Group 1
• .* - any zero or more chars other than line break chars, as many as possible
• \\1 - the same char as in Group 1.