RegExp.prototype.unicodeSets has the value true if the v flag was used; otherwise, false. The v flag is an "upgrade" to the u flag that enables more Unicode-related features. ("v" is the next letter after "u" in the alphabet.) Because u and v interpret the same regex in incompatible ways, using both flags results in a SyntaxError. With the v flag, you get all features mentioned in the u flag description, plus:

• The \p escape sequence can be additionally used to match properties of strings, instead of just characters.
• The character class syntax is upgraded to allow intersection, union, and subtraction syntaxes, as well as matching multiple Unicode characters.
• The character class complement syntax [^...] constructs a complement class instead of negating the match result, avoiding some confusing behaviors with case-insensitive matching. For more information, see Complement classes and case-insensitive matching.

Some valid u-mode regexes become invalid in v-mode. Specifically, the character class syntax is different and some characters can no longer appear literally. For more information, see v-mode character class.

The set accessor of unicodeSets is undefined. You cannot change this property directly.

• RegExp.prototype.lastIndex
• RegExp.prototype.dotAll
• RegExp.prototype.global
• RegExp.prototype.hasIndices
• RegExp.prototype.ignoreCase
• RegExp.prototype.multiline
• RegExp.prototype.source
• RegExp.prototype.sticky
• RegExp.prototype.unicode
• RegExp v flag with set notation and properties of strings on v8.dev (2022)