Regex to capture an optional group in the middle of a block of input

Some text %interestingbit% lots of random text lots and lots more %anotherinterestingbit%
Some text %interestingbit% lots of random text OPTIONAL_THING lots and lots more %anotherinterestingbit%
Some text %interestingbit% lots of random text lots and lots more %anotherinterestingbit%

%interestingbit%.+?((?<OptionalCapture>OPTIONAL_THING)).+?%anotherinterestingbit%

%interestingbit%.+?((?<OptionalCapture>OPTIONAL_THING))?.+?%anotherinterestingbit%

%interestingbit%.+?(?:(?<optionalCapture>OPTIONAL_THING).+?)?%anotherinterestingbit%

%interestingbit%.+?(?<OptionalCapture>OPTIONAL_THING)?.+?%anotherinterestingbit%

%interestingbit%.+?(?<OptionalCapture>OPTIONAL_THING|).+?%anotherinterestingbit%

%interestingbit%(?:(.+)(?<optionalCapture>OPTIONAL_THING))?(.+?)%anotherinterestingbit%