code: public void assertMatches(@Nullable Description description, @Nullable String actual, @Nonnull Pattern pattern) {
      checkNotNull(pattern);
      assertNotNull(description, actual);
      if (!pattern.matcher(actual).matches()) {
        String format = "expecting:<%s> to match regular expression:<%s>";
        throw failures.failure(description, new BasicErrorMessageFactory(format, actual, pattern.pattern()));
      }
    }
docs: Verifies that the <em>actual</em> {@code String} matches the given regular expression.
  
  @param description the description of the <em>actual</em> {@code String}.
  @param actual the <em>actual</em> {@code String}.
  @param pattern the compiled representation of the regular expression pattern to which the <em>actual</em>
  {@code String} is to be matched.
  @throws NullPointerException if the given pattern is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code String} is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code String} does not match the given regular expression.