Refactor parsePrefixAsIdd to fix String/NSString length mismatch (fixes #889)

[MarcGarciaSunweb]

fixes #889

Summary

This PR addresses a potential _String length conflation_ issue when bridging Swift `String` to Foundation APIs that operate on UTF-`16` (`NSString`/`NSRegularExpression`). The code now uses `utf16.count` when creating an `NSRange`, ensuring the range length matches what `NSRegularExpression` expects and preventing incorrect matches or out-of-bounds behavior for non-ASCII input.

Problem

Swift `String.count` returns the number of extended grapheme clusters (user-perceived characters), not the UTF-`16` code unit length used by `NSString` and `NSRegularExpression`. Using `String.count` to build an `NSRange` can cause mismatched ranges for characters represented by surrogate pairs or combining sequences, leading to incorrect regex results.

Fix

When building the `NSRange` used for regex matching, the implementation uses:

• `NSRange(location: 0, length: remainString.utf16.count)`

This aligns the range length with Foundation’s UTF-`16` indexing model.

Impact / Risk

Low risk. The change only affects the range length passed to regex evaluation and improves correctness on Unicode input. Behavior for ASCII-only input is unchanged.

Testing

• Verified behavior with existing unit tests.

Notes

This change prevents a class of issues typically flagged by static analyzers as _String length conflation_ when mixing Swift `String` indexing semantics with `NSString`/`NSRegularExpression` APIs.