Which regexp flavor is used by matches?

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From Willow in Slack:

Question: Under AttributeConstraint, the matches operator means "the value is a regular expression to be evaluated". What flavor of regular expression? For instance, JSON Schema itself uses ECMA 262 Javascript regular expressions for its schema-level patterns. But out there are also commonly-used python regex, and the defacto standard pcre2 regex. Each one has its own peculiarities and differences. Which one is the attribute constraint matches operator expected to conform to? (edited)