Case-insensitive regex matching gives culture-dependent results (Turkish "İ/i" breaks IgnoreCase)

[matt-edmondson]

What's wrong

TextFilter.cs:411-413, inside DoesMatchRegex:

RegexOptions regexOptions = caseSensitivity is TextFilterCaseSensitivity.CaseInsensitive
    ? RegexOptions.Compiled | RegexOptions.IgnoreCase
    : RegexOptions.Compiled;

RegexOptions.IgnoreCase without RegexOptions.CultureInvariant folds case using the current thread culture, not an invariant/ordinal fold. Under a Turkish culture (tr-TR), uppercase I and lowercase i are not case-equivalent (the dotted/dotless I problem: i folds to İ, not I).

Concrete failure scenario

CultureInfo.CurrentCulture = new CultureInfo("tr-TR");
new Regex("i", RegexOptions.IgnoreCase).IsMatch("I");   // False
new Regex("I", RegexOptions.IgnoreCase).IsMatch("i");   // False
// same pattern with RegexOptions.CultureInvariant added -> True

So TextFilter.DoesMatchRegex("IMG_1234.JPG", "img", opts, TextFilterCaseSensitivity.CaseInsensitive) returns a different answer depending on the calling thread's CurrentCulture — a silent, hard-to-reproduce bug for any app whose UI thread runs under a Turkish locale (not an exotic case). This is a live gap in newly-added code: the case-insensitive matching feature was added in commit 7b59874 (closing #97), and neither the README's "Case Sensitivity" section nor the tests around TextFilterTests.cs:582-625 mention or test culture, so this isn't a documented/intentional limitation.

The glob path (DotNet.Glob's CaseInsensitive option, used in ResolveGlob) does not have this problem — it matches correctly regardless of culture. The bug is isolated to the regex branch.

Suggested fix

Add RegexOptions.CultureInvariant alongside RegexOptions.IgnoreCase at line 412:

RegexOptions regexOptions = caseSensitivity is TextFilterCaseSensitivity.CaseInsensitive
    ? RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.CultureInvariant
    : RegexOptions.Compiled;

Add a regression test that sets CurrentCulture to tr-TR (or uses CultureInfo.InvariantCulture vs tr-TR explicitly) and asserts img matches IMG_1234.JPG case-insensitively regardless of culture.