java - Why does Apache Commons consider '???' numeric?

Question

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

Answer 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.

??? are Devanagari digits:

• ? is DEVANAGARI DIGIT ONE, u0967
• ? is DEVANAGARI DIGIT TWO, u0968
• ? is DEVANAGARI DIGIT THREE, u0969