fleshing out

README.md

@@ -1,27 +1,70 @@
 cronexpression for Go
 =====================
+Cron expression parser in Go language (golang).
 
-Go language (golang) cron expression parser. Given a cron expression and a time stamp, you can get the next time stamp which satisfy the cron expression.
+Given a cron expression and a time stamp, you can get the next time stamp which satisfy the cron expression.
 
+Implementation
+--------------
 The reference documentation for this implementation is found at
-https://en.wikipedia.org/wiki/Cron#CRON_expression, with the following
-differences:
+https://en.wikipedia.org/wiki/Cron#CRON_expression, which I copy/pasted here (laziness) with modifications where this implementation differs:
 
-* Supports the second field (before minute field)
+    Field name     Mandatory?   Allowed values    Allowed special characters
+    ----------     ----------   --------------    --------------------------
+    Minutes        Yes          0-59              * / , -
+    Hours          Yes          0-23              * / , -
+    Day of month   Yes          1-31              * / , - ? L W
+    Month          Yes          1-12 or JAN-DEC   * / , -
+    Day of week    Yes          0-6 or SUN-SAT    * / , - ? L #
+    Year           No           1970–2099         * / , -
+
+Asterisk ( * )
+--------------
+The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month. 
+
+Slash ( / )
+-----------
+Slashes describe increments of ranges. For example `3-59/15` in the minute field indicate the third minute of the hour and every 15 minutes thereafter. The form `*/...` is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field.
+
+Comma ( , )
+-----------
+Commas are used to separate items of a list. For example, using `MON,WED,FRI` in the 5th field (day of week) means Mondays, Wednesdays and Fridays.
+
+Hyphen ( - )
+------------
+Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.
+
+L
+-
+`L` stands for "last". When used in the day-of-week field, it allows you to specify constructs such as "the last Friday" (`5L`) of a given month. In the day-of-month field, it specifies the last day of the month.
+
+W
+-
+The `W` character is allowed for the day-of-month field. This character is used to specify the weekday (Monday-Friday) nearest the given day. As an example, if you were to specify `15W` as the value for the day-of-month field, the meaning is: "the nearest weekday to the 15th of the month." So, if the 15th is a Saturday, the trigger fires on Friday the 14th. If the 15th is a Sunday, the trigger fires on Monday the 16th. If the 15th is a Tuesday, then it fires on Tuesday the 15th. However if you specify `1W` as the value for day-of-month, and the 1st is a Saturday, the trigger fires on Monday the 3rd, as it does not 'jump' over the boundary of a month's days. The `W` character can be specified only when the day-of-month is a single day, not a range or list of days.
+
+Hash ( # )
+----------
+`#` is allowed for the day-of-week field, and must be followed by a number between one and five. It allows you to specify constructs such as "the second Friday" of a given month.
+
+Question mark ( ? )
+-------------------
+Note: Question mark is a non-standard character and exists only in some cron implementations. It is used instead of `*` for leaving either day-of-month or day-of-week blank.
+
+With the following differences:
+
+* Supports optional second field (before minute field)
 * If five fields are present, a wildcard year field is appended
-* If six field are present, "0" is prepended as second field
-* Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday, like zero.
-* `@reboot` is not supported, as it is meaningless for a cron expression parser library
-* As of now, the behavior of the code is undetermined if a malformed cron expression is supplied (most likely, code will panic)
+* If six field are present, `0` is prepended as second field, that is, `* * * * * *` internally become `0 * * * * * *`.
+* Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0).
+* `@reboot` is not supported
+* As of now, the behavior of the code is undetermined if a malformed cron expression is supplied
 
 Install
 -------
-
     go get github.com/gorhill/cronexpression
 
 Usage
 -----
-
 Import the library:
 
     import "github.com/gorhill/cronexpression"