Binary properties are a kind of Unicode property that flavors may support. They are called binary because a code point either has the property or not. The 44 characters in the string 0123456789ABCDEFabcdef０１２３４５６７８９ＡＢＣＤＥＦａｂｃｄｅｆ all have the Hex_Digit property. Every other code point does not have the property. Unassigned code points also don’t have the property. So \p{Hex_Digit} matches any of these 44 characters, while \P{Hex_Digit} matches every other code point, including unassigned ones.

The \p{Property_Name} syntax is supported by ICU, Perl, and JavaScript with /u. Ruby has supported binary properties since version 1.9. PCRE2 added support for binary properties in version 10.40. Because they are based on PCRE2, PHP adds it with version 8.2.0 and R with version 4.2.2.

A binary property can also be seen as a property set with two possible values: Yes and No. The 44 characters in the string above have the value Yes for the property Hex_Digit, while all other code points have the value No. ICU and Perl support the property set syntax for binary properties. \p{Hex_Digit=Yes} matches the 44 hex digits while \p{Hex_Digit=No} matches all other code points. You may find the latter more readable than the negated property \P{Hex_Digit}. Be mindful of double negation. \P{Hex_Digit=No} matches all code points that have Hex_Digit != No which are the 44 code points in the sample string. Thus \P{Hex_Digit=No} is identical to \p{Hex_Digit=Yes}.

Most binary properties have a long canonical name and a shorter alias. The Hex_Digit property has Hex as its alias. The Yes and No values also have Y and N as aliases. So you can write \p{Hex} or \p{Hex=Y}.

Unicode provides quite the list of binary properties. New Unicode versions regularly add new properties in support of new Unicode features. None of the flavors discussed here support all binary properties. The fact that a property exists in a certain Unicode version and that your regex flavor is based on that Unicode version doesn’t mean that the flavor supports that property. See the Unicode binary property reference for a complete list of all Unicode properties and which versions of which regex flavors support them.