Why does Apache Commons consider '१२३' numeric?

问题

According to Apache Commons Lang's documentation for StringUtils.isNumeric(), the String '१२३' is numeric.

Since I believed this might be a mistake in the documentation, I ran tests to verify the statement. I found that according to Apache Commons it is numeric.

Why is this String numeric? What do those characters represent?

回答1:

Because that "CharSequence contains only Unicode digits" (quoting your linked documentation).

All of the characters return true for Character.isDigit:

Some Unicode character ranges that contain digits:

'\u0030' through '\u0039', ISO-LATIN-1 digits ('0' through '9')

'\u0660' through '\u0669', Arabic-Indic digits

'\u06F0' through '\u06F9', Extended Arabic-Indic digits

'\u0966' through '\u096F', Devanagari digits

'\uFF10' through '\uFF19', Fullwidth digits

Many other character ranges contain digits as well.

१२३ are Devanagari digits:

१ is DEVANAGARI DIGIT ONE, \u0967
२ is DEVANAGARI DIGIT TWO, \u0968
३ is DEVANAGARI DIGIT THREE, \u0969

回答2:

The symbol १२३ is the same as 123 for the Nepali language or any other language using the Devanagari script such as Hindi, Gujarati, and so on, and is therefore is a number for Apache Commons.

回答3:

You can use Character#getType to check the character's general category:

System.out.println(Character.DECIMAL_DIGIT_NUMBER == Character.getType('१'));

This will print true, which is an "evidence" that '१' is a digit number.

Now let's examine the unicode value of the '१' character:

System.out.println(Integer.toHexString('१'));
// 967

This number is on the range of Devanagari digits - which is: \u0966 through \u096F.

Also try:

Character.UnicodeBlock block = Character.UnicodeBlock.of('१');
System.out.println(block.toString());
// DEVANAGARI

Devanagari is:

is an abugida (alphasyllabary) alphabet of India and Nepal

"१२३" is a "123" (Basic Latin unicode).

Reading:

More details on the '१' character
StringUtils#isNumeric implementation

回答4:

If you ever want to know what properties a particular "character" has (and there are quite a few), go directly to the source: Unicode.org. They have research tools that can show you most anything you would care to know.

If you want to see all of the properties of a specific character, try the following:

http://unicode.org/cldr/utility/character.jsp?a=१

or:

http://unicode.org/cldr/utility/character.jsp?a=%E0%A5%A7
If you want to see all characters classified as "decimal digits" (i.e. with number values of 0 through 9), try the following:

http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[:Numeric_Type=Decimal:]
^{( 550 Code Points -- currently / as of Unicode 9.0 )}
If you want to see all characters classified as "non-decimal digit numbers" (i.e. fractions, circled, etc), try the following:

http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[:Numeric_Type=Numeric:]
^{( 836 Code Points -- currently / as of Unicode 9.0 )}
If you want to see all characters classified as "decimal digits" (i.e. with number values of 0 through 9), but only up through Unicode 6.0 (which .NET uses), try the following:

http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[:Numeric_Type=Decimal:]%26[:Age=6.0:]
^{( 420 Code Points -- and shouldn't change )}
If you want to see all characters classified as "decimal digits" (i.e. with number values of 0 through 9), but only up through Unicode 6.0 (which .NET uses), and only in the Base-Multilingual Plane / no Supplementary Characters (i.e. nothing above Code Point 65535 / U+0xFFFF), try the following:

http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[:Numeric_Type=Decimal:]%26[:Age=6.0:]%26[:bmp=Yes:]
^{( 350 Code Points -- and shouldn't change )}

KEEP IN MIND: The Unicode Consortium produces a specification, not software. This means that it is up to each software vendor to implement the specification as accurately as they can. So just like HTML, JavaScript, CSS, SQL, etc, there is variation between different platforms, languages, and so on. For example, I found a bug in Microsoft's .NET Framework whereby circled Latin letters A-Z and a-z -- Code Points 0x24B6 through 0x24E9 -- do not properly register as being char.IsLetter = true (bug report here). And that leads to unexpected behavior in related functionality, such as when calling the TextInfo.ToTitleCase() method (bug report here).