[wmundev]

NormalizeDigitsOnly("\u0BF1123") throws on modern targets because Tamil hundred expands into three ASCII digits inside an input-sized buffer. Other numeric symbols can also introduce digits that Java excludes.

Use Unicode decimal-digit values in both the span and StringBuilder implementations, matching Java's Character.digit(c, 10). Each accepted character now produces one ASCII digit; non-decimal numeric characters are stripped or retained according to keepNonDigits.

Regression coverage includes Tamil hundred, fractions, Roman numerals, superscripts, circled numbers, and a negative numeric value. It also verifies Arabic-Indic, Devanagari and fullwidth decimal digits, both stack and pooled inputs, and the shared StringBuilder helper.

Upstream behavior: https://github.com/google/libphonenumber/blob/33be5664343676b27761213d8f5500d682568ea7/java/libphonenumber/src/com/google/i18n/phonenumbers/PhoneNumberUtil.java#L825-L837

Validation:

• Confirmed the non-decimal regression fails before the fix; decimal-digit preservation passes before and after.
• Full solution test matrix: 998 passing executions across .NET 8 and .NET 10.
• All target frameworks build with zero warnings and errors.
• Robustness tests run directly against netstandard2.0 on both runtimes: 20 passing executions for this branch.
• All three fixes apply together without conflicts; combined matrix passes 1,006 executions, plus 28 robustness executions directly against netstandard2.0.
• BenchmarkDotNet ParseOnly comparison completed: allocation stayed at 348.13 KB per 1,000 numbers. Short timing samples were too noisy to establish a throughput change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.91%. Comparing base (387caaf) to head (81b25cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
- Coverage   87.92%   87.91%   -0.02%     
==========================================
  Files          43       43              
  Lines        3900     3896       -4     
  Branches      995      995              
==========================================
- Hits         3429     3425       -4     
  Misses        272      272              
  Partials      199      199              

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