How to I validate a string to ensure it contains a valid Date .

Question

Matt O'Brien 2023-3-11

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编辑： Stephen23 2023-3-11

I know how to convert a string to a date...... but how do I validate if the string is a valid date.

Example

MyStr = '23max2023'; % invalid may typo max
infmt = "ddMMMyyyy";
MyDate = datetime(MyStr,"InputFormat",infmt)
This generates an error if invalid.... is there an elegant method to check if a date is valid without having to generate an error and checking the type of error.
    

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Answer 1

the cyclist 2023-3-11

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编辑：the cyclist 2023-3-11

在 MATLAB Online 中打开

Here is one way:

MyStr = '23max2023'; % Invalid string will give an empty array
isMyDateFormat = regexp(MyStr, '\d{2}(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d{4}')
isMyDateFormat =

     []
MyStr = '23may2023'; % Valid string will give a 1
isMyDateFormat = regexp(MyStr, '\d{2}(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d{4}')
isMyDateFormat = 1

This method specifically check for the pattern of exactly 2 digits, then a lower-case standard month abbreviation, then exactly 4 digits. One could get even more sophisticated if needed.

Alternatively, you could wrap your code in a try-catch structure, to try the datetime command, then do something else if it fails:

MyStr = '23max2023';
infmt = "ddMMMyyyy";
try
    MyDate = datetime(MyStr,"InputFormat",infmt)
catch
    fprintf("Invalid date format")
end
Invalid date format

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Stephen23 2023-3-11

编辑：Stephen23 2023-3-11

在 MATLAB Online 中打开

"I used the lower function to create a string with all lowercase alpha chars."

Simpler and more efficient: use REGEXPI.

"I then checked that the results of the regexp has a length of 0"

Note that this is not robust (nor is the provided answer) to prefixed/suffixed text:

rgx = '\d{2}(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d{4}';
regexpi('NotValidDate23Jan2023NorThis', rgx)
ans = 13
regexpi('012345678923Jan2023123456789', rgx)
ans = 11

You can ensure the entire text is checked by using ^ and $:

rgx = '^\d{2}(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d{4}$';
regexpi('23Jan2023', rgx)
ans = 1
regexpi('012345678923Jan2023123456789', rgx)
ans =

     []

In practice defining booleans as negative of something is usually a bad approach (it leads to confusion in writing the code later), so we define it as the positive statement:

IsValidDate = isscalar(regexpi('23Jan2023', rgx))
IsValidDate = logical
   1

Rather than trying to reverse engineer the date validation, I would recommend using the TRY/CATCH approach.

Steven Lord 2023-3-11

在 MATLAB Online 中打开

The regexp based approach will work, as long as you don't need to deal with month names in a different locale as shown in the "Date and Time from Text in Foreign Language" example on the datetime documentation page. Looking at the try / catch based approach from @the cyclist's original answer with a bit of modification:

MyStr = '23max2023';
infmt = "ddMMMyyyy";
try
    MyDate = datetime(MyStr,"InputFormat",infmt)
catch ME
    fprintf("Invalid date format. Error ID:\n%s\nError message:\n%s\n", ...
        ME.identifier, ME.message)
end
Invalid date format. Error ID:
MATLAB:datetime:ParseErrSuggestLocale
Error message:
Unable to convert '23max2023' to datetime using the format 'ddMMMyyyy'. If the date/time text contains day, month, or time zone names in a language foreign to the 'en_US' locale, those might not be recognized. You can specify a different locale using the 'Locale' parameter.

Compare this with a format that is completely mismatched with the date string:

MyStr = '23max2023';
infmt = "ddyyyy";
try
    MyDate = datetime(MyStr,"InputFormat",infmt)
catch ME
    fprintf("Invalid date format. Error ID:\n%s\nError message:\n%s\n", ...
        ME.identifier, ME.message)
end
Invalid date format. Error ID:
MATLAB:datetime:ParseErr
Error message:
Unable to convert '23max2023' to datetime using the format 'ddyyyy'.

The first error identifier suggests that you misspelled the month abbreviation (or you're using an abbreviation in a different locale.) The second suggests a larger error. You could check which error was thrown and handle those two cases differently (and if something else happened, rethrow the error and let the calling code handle it or not as appropriate.)

Matt O'Brien 2023-3-11

Thank you. I will use this approach. I understand and agree with the approach of not re-engineering a date validation function.

To me, a recent MatLab coder, it is staggering to think that the following format does not exist...

[ MyDateVal, MyDateStatus] = datetime(.............

Anyway...I will use the try ... catch syntax.

All feedback appreciated.

I need to figure out how to flag this to be the correct answer.

Steven Lord 2023-3-11

在 MATLAB Online 中打开

To me, a recent MatLab coder, it is staggering to think that the following format does not exist...

[ MyDateVal, MyDateStatus] = datetime(.............

If datetime behaved that way instead of throwing an error, every single call to datetime would need to be followed by an if call checking that MyDateStatus indicated success before proceeding to use MyDateVal. With the error-based approach, as long as the next line runs the datetime call succeeded.

I wouldn't want to see code that was forced to look like the following, to take the "return result and status" pattern to the extreme:

x = 1:10;
y = 11;
[result, didSucceeed] = plus(x, y);
if ~didSucceed % We couldn't add x and y
    % handle the error
end

IMO better to just let plus (or datetime) throw the error itself. The function has more information about what exactly went wrong, so it may be able to distinguish between the "you gave me a month name I didn't recognize but that could be a valid month name in a different locale" and "your data doesn't match the format you asked for at all" cases and provide more specifically tailored messages.

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