This is the same problem people have with closures, where it's unclear to the user whether the argument is captured by name or by value.

This isn't the same problem, because this is about whether the regex is instantiated each time the code around the regex is executed, or only the first time and cached for subsequent executions. The same could in theory happen with closures, but I haven't ever seen programming-language semantics where, for example, a function containing the definition of a closure that depends on an argument of that outer function, would use the argument value of the first invocation of the function for all subsequent invocations of the function.

For example, when you have

    fn f x = (y -> x + y)

    f 1 3
    f 2 6

The parallel is apt, but regex /o is more like a closure that captures by value at declaration time rather than an ambiguity between capture strategies.