Cleanup PR based on feedback

- Optimize scenario where only 'L' modifier exists in the wday expression to
  reduce the number of invocations of the calendar module.
- Cleanup and combined parsing logic for specialty wday syntax (d#n/ Ld)
- Add new unit tests to confirm proper exception handling for out-of-range 'L'
  and '#' nth month for the wday expression.
- Add examples of new syntax in README.
- Update changelog to cover new syntax and new exception.

4 files changed, 49 insertions, 19 deletions

diff --git a/README.rst b/README.rst
index d658874..7ecdca0 100644
--- a/README.rst
+++ b/README.rst
@@ -50,8 +50,13 @@ A simple example::
     >>> print(iter.get_next(datetime))   # 2010-09-01 04:02:00
     >>> print(iter.get_next(datetime))   # 2010-12-01 04:02:00
     >>> print(iter.get_next(datetime))   # 2011-06-01 04:02:00
+    >>>
     >>> iter = croniter('0 0 * * sat#1,sun#2', base)
     >>> print(iter.get_next(datetime))   # datetime.datetime(2010, 2, 6, 0, 0)
+    >>> iter = croniter('0 0 * * 5#3,L5', base) # 3rd and last Fridays of the month
+    >>> print(iter.get_next(datetime))   # 2010-01-29 00:00:00
+    >>> print(iter.get_next(datetime))   # 2010-02-19 00:00:00
+
 
 All you need to know is how to use the constructor and the ``get_next``
 method, the signature of these methods are listed below::
diff --git a/docs/CHANGES.rst b/docs/CHANGES.rst
index 2ef4104..7ef9e9d 100644
--- a/docs/CHANGES.rst
+++ b/docs/CHANGES.rst
@@ -4,7 +4,14 @@ Changelog
 1.0.11 (unreleased)
 -------------------
 
-- Nothing changed yet.
+- Add support for ``L`` in the day_of_week component.  This enable expressions like ``* * * * L4``, which means last Thursday of the month.  This resolves #159.
+  [Kintyre]
+- Create ``CroniterUnsupportedSyntaxError`` exception for situations where CRON syntax may be valid but some combinations of features is not supported.
+  Currently, this is used when the ``day_of_week`` component has a combination of literal values and nth/last syntax at the same time.
+  For example, ``0 0 * * 1,L6`` or ``0 0 * * 15,sat#1`` will both raise this exception because of mixing literal days of the week with nth-weekday or last-weekday syntax.
+  This *may* impact existing cron expressions in prior releases, because ``0 0 * * 15,sat#1`` was previously allowed but incorrectly handled.
+  [Kintyre]
+
 - Update ``croniter_range()`` to allow an alternate ``croniter`` class to be used.  Helpful when using a custom class derived from croniter.
   [Kintyre]
 
diff --git a/src/croniter/croniter.py b/src/croniter/croniter.py
index c2f6e1d..17e883d 100644
--- a/src/croniter/croniter.py
+++ b/src/croniter/croniter.py
@@ -17,7 +17,7 @@ import calendar
 step_search_re = re.compile(r'^([^-]+)-([^-/]+)(/(\d+))?$')
 only_int_re = re.compile(r'^\d+$')
 star_or_int_re = re.compile(r'^(\d+|\*)$')
-last_weekday_re = re.compile(r'^l([0-7])$')
+special_weekday_re = re.compile(r'^(\w+)#(\d+)|l(\d+)$')
 VALID_LEN_EXPRESSION = [5, 6]
 
 
@@ -394,24 +394,24 @@ class croniter(object):
 
             candidates = []
             for wday, nth in nth_weekday_of_month.items():
-                # XXX: Optimize to only run calendar, when needed:  if len(nth) > 1 or nth[0] != "l":
-                w = (wday + 6) % 7
-                c = calendar.Calendar(w).monthdayscalendar(d.year, d.month)
-                if c[0][0] == 0:
-                    c.pop(0)
+                c = None
+                if nth != {"l"}:
+                    w = (wday + 6) % 7
+                    c = calendar.Calendar(w).monthdayscalendar(d.year, d.month)
+                    if c[0][0] == 0:
+                        c.pop(0)
                 for n in nth:
                     if n == "l":
                         candidate = self._get_last_weekday_of_month(d.year, d.month, wday)
+                    elif len(c) < n:
+                        continue
                     else:
-                        if len(c) < n:
-                            continue
                         candidate = c[n - 1][0]
                     if (
                         (is_prev and candidate <= d.day) or
                         (not is_prev and d.day <= candidate)
                     ):
                         candidates.append(candidate)
-                del w, c
 
             if not candidates:
                 if is_prev:
@@ -592,18 +592,24 @@ class croniter(object):
 
                 if i == 4:
                     # Handle special case in the day-of-week expression
-                    m = last_weekday_re.match(str(e))
+                    m = special_weekday_re.match(str(e))
                     if m:
-                        e = m.group(1)
-                        nth = "l"
-                    else:
-                        e, sep, nth = str(e).partition('#')
+                        orig_e = e
+                        e, nth, last = m.groups()
                         if nth:
-                            if not re.match(r'[1-5]', nth):
+                            try:
+                                nth = int(nth)
+                                assert (nth >= 1 and nth <= 5)
+                            except (ValueError, AssertionError):
                                 raise CroniterBadCronError(
-                                    "[{0}] is not acceptable".format(expr_format))
-                            nth = int(nth)
-                        del sep
+                                    "[{0}] is not acceptable.  Invalid day_of_week "
+                                    "value: '{1}'".format(expr_format, orig_e))
+                        elif last:
+                            nth = "l"
+                            e = last
+                        del last, orig_e
+                    else:
+                        nth = None
 
                 # Before matching step_search_re, normalize "*" to "{min}-{max}".
                 # Example: in the minute field, "*/5" normalizes to "0-59/5"
diff --git a/src/croniter/tests/test_croniter.py b/src/croniter/tests/test_croniter.py
index fd82580..c171983 100755
--- a/src/croniter/tests/test_croniter.py
+++ b/src/croniter/tests/test_croniter.py
@@ -1308,6 +1308,18 @@ class CroniterTest(base.TestCase):
         self.assertListEqual(getn(cron_b, 3), expect_b)
         self.assertListEqual(getn(cron_c, 5), expect_c)
 
+    def test_nth_out_of_range(self):
+        with self.assertRaises(CroniterBadCronError):
+            croniter("0 0 * * 1#7")
+        with self.assertRaises(CroniterBadCronError):
+            croniter("0 0 * * 1#0")
+
+    def test_last_out_of_range(self):
+        with self.assertRaises(CroniterBadCronError):
+            croniter("0 0 * * L-1")
+        with self.assertRaises(CroniterBadCronError):
+            croniter("0 0 * * L8")
+
     def test_issue_142_dow(self):
         ret = []
         for i in range(1, 31):