code: public @Nonnull
    StringAssert matches(@Nonnull String regex) {
      strings.assertMatches(description, actual, regex);
      return this;
    }
docs: Verifies that the <em>actual</em> {@code String} matches the given regular expression.
  
  @param regex the regular expression to which the <em>actual</em> {@code String} is to be matched.
  @return {@code this} assertion object.
  @throws NullPointerException if the given pattern is {@code null}.
  @throws PatternSyntaxException if the regular expression's syntax is invalid.
  @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.