How to handle combining characters along with the \p{L} pattern for Thai strings?

Question

I need to detect text with Unicode characters restricting it to letters only (e.g. no symbols, emojis, etc., just something that can be used in a person's name in any Unicode language). The \p{L} category seems to do the trick, but it does not recognize Thai strings. I do not speak Thai, so I got a few common Thai names from ChatGPT and they all fail in my test. Tried it at RegExr (see the Tests tab) and also wrote a simple test program:

using System.Text.RegularExpressions;

Console.OutputEncoding = System.Text.Encoding.UTF8;

string pattern = @"^[\p{L}\s]+$";

string englishText = "Mary";
Console.Write($"{englishText}: ");
Console.WriteLine(Regex.IsMatch(englishText, pattern, RegexOptions.IgnoreCase).ToString()); // true

string germanText = "RöschenÜmit";
Console.Write($"{germanText}: ");
Console.WriteLine(Regex.IsMatch(germanText, pattern, RegexOptions.IgnoreCase).ToString()); // true

string thaiText = "อรุณรัตน์";
Console.Write($"{thaiText}: ");
Console.WriteLine(Regex.IsMatch(thaiText, pattern, RegexOptions.IgnoreCase).ToString()); // false

string japaneseText = "タクミたくみく";
Console.Write($"{japaneseText }: ");
Console.WriteLine(Regex.IsMatch(japaneseText, pattern, RegexOptions.IgnoreCase).ToString()); // true

I noticed when I try testing each individual character in the Thai string, it seems to recognize them as valid Unicode letters, but as a string, it fails. Just to make sure I do not have any hidden characters, I checked the raw values and I did not see anything suspicious. Any ideas what's going on here?

P.S. I know that some of the characters in the test are from different sets and names may include spaces, dashes, etc., but this is not the point. I'm just trying to solve the Thai strings issue here.

COMMENT: Thai string contains combining character which I guess causes the problem in detecting letters even if those look as single letter (i.e. {0e23, 0xe38} results in "รุ").

Answer 1

If we print out thaiText dump:

string thaiText = "อรุณรัตน์";

var report = string.Join(Environment.NewLine, thaiText
  .Select(c => $"{c} : \\u{(int)c:x4} : {char.GetUnicodeCategory(c)}"));

Console.WriteLine(report);

We'll get the cause of misbehaviour: NonSpacingMarks category between the OtherLetters:

อ : \u0e2d : OtherLetter
ร : \u0e23 : OtherLetter
ุ : \u0e38 : NonSpacingMark <- doesn't match
ณ : \u0e13 : OtherLetter
ร : \u0e23 : OtherLetter
ั : \u0e31 : NonSpacingMark <- doesn't match
ต : \u0e15 : OtherLetter
น : \u0e19 : OtherLetter
์ : \u0e4c : NonSpacingMark <- doesn't match

Technically, to get rid of these marks we can use normalization:

// The idea is to combine marks and letters into a letter which should match
thaiText = thaiText.Normalize(NormalizationForm.FormD);

but it doesn't work at my workstation and the reason is an issue

So if normalization doesn't work in your case as well (or you want to be on the safer side of the road), you can try match Thai symbols; either only Thai

string pattern = @"^[\p{IsThai}\s]+$";

or mixing with all the other ones (letters or Thai letters as a special case):

string pattern = @"^[\p{L}\p{IsThai}\s]+$";

or allow both letters (\p{L}) and these non-spacing marks (\p{Mn}):

string pattern = @"^[\p{L}\p{Mn}\s]+$";