Regex whole word match not working for Vietnamese language

Good evening!

I have a string field in Mongo which holds text in Vietnamese. For example, 2 documents:

  1. bắt
  2. bắt cóc

The program needs to do a whole “word” match against the text. Link to regex101 website with a demo working as expected. E.g…

bắt → return #1 and #2 above
cóc → return #2 above
bắ → return nothing
có → return nothing

Here’s the .NET code, but the word boundary \b is not working. E.g. using “bắ”

string syllable = "bắ";
string pattern = @"\b" + syllable + @"\b";

var options = RegexOptions.CultureInvariant | RegexOptions.IgnoreCase;
var regex = new Regex(pattern, options);
var filter = Builders<Translation>.Filter.Regex(t => t.Vietnamese, new BsonRegularExpression(regex));

var results = collection

Expected: No results
Actual: Both 1. bắt and 2. bắt cóc are returned in error

The same issue occurs querying directly through Atlas / Compass. E.g.

{ Vietnamese: { $regex: /\bcó\b/, $options: "ui" } }

Expected: No results
Actual: #2. bắt cóc is returned in error

It has something to do with encoding. When I run the same code against data with only Basic Latin characters, it works as expected.


Fixed using “negative lookbehind” and “negative lookahead.” :smiley:

The same goal to have a whole “word” match. The negative lookbehind ensures the match is not preceded by anything except a space, if any. The negative lookahead ensures the match is not succeeded by anything except a space, if any.


E.g. bắ

var regex = new Regex(@"(?<!\S)" + match + @"(?!\S)", RegexOptions.IgnoreCase);

Does this look like the same issue mentioned in this ticket?

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Yes, that looks like it! \b unicode issue

Thanks for finding that Asya.

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