Fix "MMM.yy" date parsing in German culture. #114169

[pra2892]

This solution handles all the edge cases:

• The FormatContainsDayOfMonthSpecifier method properly skips 'd' characters inside quotes.
• Both \ and % escape characters are handled.
• We only consider 'd' or 'dd' as day-of-month specifiers, not 'ddd' or 'dddd' (day-of-week).
• We analyze the entire format string, so it doesn't matter if 'MMM' comes before or after 'd'.
• The entire format is analyzed, so multiple day or month patterns are handled correctly.
• We only scan the format string once, and only when needed.

This solution should work correctly in all scenarios, including the specific case mentioned with "MMM dd" format in German culture.

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[lilinus]

It would probably be good with some tests for this kind of case.

[tarekgh]

@pra2892 thanks again for submitting the PR. This looks good. We need to add some tests to ensure we'll not break the behavior in the future. I would suggest in the test you create a culture like the following and ensure setting culture.DateTimeFormat.AbbreviatedMonthGenitiveNames to guarantee the test will never fail in the future if the culture data changed.

 var culture = new CultureInfo("de-DE");
culture.DateTimeFormat.AbbreviatedMonthGenitiveNames = ["Jan.", "Feb.", "März", "Apr.", "Mai", "Juni", "Juli", "Aug.", "Sept", "Okt.", "Nov.", "Dez.", ""];
culture.DateTimeFormat.MonthGenitiveNames = ["Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember.", ''"];
culture.DateTimeFormat.DayNames = ["Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag"];
culture.DateTimeFormat.DateSeparator = ".";

Then you add test cases:

• Parse exact Dez.20 with the format MMM.yy and ensure the parsing succeeds.
• parse exact Dez.20 01 with the format MMM.yy dd and ensure the parsing fails.
• parse exact 01 Dez.20 with the format d MMM.yy and ensure the parsing fails.
• parse exact Dienstag Dez.20 with the format dddd MMM.yy and ensure the parsing succeed.
• then repeat the same tests with patterns using MMMM and month name Dezember.

You need to add the tests to the file https://github.com/dotnet/runtime/blob/main/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs

[ANahr]

Two small "corrections"/improvements (especially "Sept." because it is the only one with 5 chars):

var culture = new CultureInfo("de-DE");
culture.DateTimeFormat.AbbreviatedMonthGenitiveNames = ["Jan.", "Feb.", "März", "Apr.", "Mai", "Juni", "Juli", "Aug.", "Sept.", "Okt.", "Nov.", "Dez.", ""];
culture.DateTimeFormat.MonthGenitiveNames = ["Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember", ''"];
culture.DateTimeFormat.DayNames = ["Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag"];
culture.DateTimeFormat.DateSeparator = ".";

[tarekgh]

@ANahr I intentionally used dot at the end of the Dezember. so we can successfully test with the date patterns we are using. Fixing any other month names other than December is irrelevant if we are always testing with December.

[tarekgh]

@pra2892 did you have a chance to address my comment #114194 (review)?

[tarekgh]

@pra2892 I have pushed the test fix to this PR so we can merge it.

[tarekgh]

/ba-g the failure is already tracked with #114465