Which string functions are considered predicate functions?

Predicate functions are specifically designed to evaluate conditions and return a boolean value, indicating whether a certain condition is met for the string being processed. In the context of this question, the function that is categorized as a predicate function is the one that checks for a specific pattern or condition within a string.

The matches function stands out as a predicate function because it checks whether a given string matches a specified regex pattern. This is a clear evaluation leading to a true or false outcome, thereby qualifying it as a predicate.

The other string functions listed—find_any_not_of, count_any_of, and contains_any_of—are not predicate functions because they do not return a simple true or false value about whether a condition is satisfied; rather, they perform operations that yield numerical results or serve other purposes. For instance, they may return the count of occurrences or the position of substrings but do not inherently express a condition's fulfillment in the binary manner characteristic of predicate functions.