regexParser
import unit_threaded; auto res = regex("(a)(.)(c)").parse("abcd"); res.success.shouldBeTrue; res.results.shouldEqual(["abc", "a", "b", "c"]); res.rest.shouldEqual("d");
regexParser works from the start of the input
import unit_threaded; auto res = regex("abc").parse("babc"); res.success.shouldBeFalse;
convenient function to instantiate a regexparser