Why is `\d` Less Efficient Than `[0-9]` for Digit Matching in C# Regex?

C#regular expression

The reason for is lower than d [0-9]

The recent performance test shows that in the C#regular expression engine, the efficiency used to match the number of

is unexpectedly lower than the d and [0-9] character set. This discovery triggered two problems: [0123456789]

<.> 1. Why is the range

higher efficiency than setting ? [0-9] [0123456789] People may think that the scope is more efficient, because it covers a specific and narrow character range, and the collection clearly lists all ten numbers. However, the test results show that the performance differences between the two methods are the least.

<.> 2. Why

The efficiency of

is lower than

? d Test found that the efficiency of is significantly lower than [0-9]. This is particularly puzzling, because is usually considered as abbreviation of

. Further investigation reveals a fundamental difference:

d [0-9] matching unicode numbers, d matching ASCII numbers [0-9]

with only ASCII numbers (0-9), d match [0-9] all unicode numbers . This includes characters from other languages ​​and characters, such as Persian numerals (۱۲۳۴۵۶۷۸۹) and Dava Garri numerals (०१२३४५६७८९). To prove this, the following code generates a string containing all Unicode numbers:

[0-9] The generated string shows d various characters identified as numbers, including characters from Arabic, Thai, Khamno, and other characters. Performance impact

<code class="language-csharp">var sb = new StringBuilder();
for (UInt16 i = 0; i < 0x10FFFF; i++)
{
    if (char.IsDigit((char)i))
    {
        sb.Append((char)i);
    }
}
string unicodeDigits = sb.ToString();</code>

Check the wide range of character range (unicode numbers and ASCII numbers) explaining the performance differences between d and

. In the case of only ASCII numbers, use

will produce better performance than .

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