# date.script
# define date expressions from "Monday, January 1, 1" to "Sunday,
# December 31, 9999" and build a shallow parser to mark them

define OneToNine [1|2|3|4|5|6|7|8|9];
define ZeroToNine ["0"|OneToNine];
define Even [{0} | 2 | 4 | 6 | 8] ;
define Odd [1 | 3 | 5 | 7 | 9] ;
define N [Even | Odd];

define Day     [{Monday} | {Tuesday} | {Wednesday} | {Thursday} |
                {Friday} | {Saturday} | {Sunday}] ;

define Month29 {February};
define Month30 [{April} | {June} | {September} | {December}];
define Month31 [{January} | {March} | {May} | {July} | {August} |
                {October} | {December}] ;

define Month   [Month29 | Month30 | Month31];

# Numbers from 1 to 31
define Date    [OneToNine | [1 | 2] ZeroToNine | 3 [%0 | 1]] ;

# Numbers from 1 to 9999
define Year [OneToNine ZeroToNine^<4];

# Day or [Month and Date] with optional Day and Year
define AllDates [Day | (Day {, }) Month { } Date ({, } Year)];

# Constraints on dates 30 and 31

define MaxDays30 ~$[Month29 { 30}];
define MaxDays31 ~$[[Month29 | Month30] { 31}];

# Combining constraints on dates 30 and 31
define MaxDays  [MaxDays30 & MaxDays31];

# Divisible by 4
# Of single digit numbers, 4 and 8 are divisible by 4.
# In larger numbers divisible with 4, if the penultimate
# is even, the last number is 0, 4, or 8. If the penultimate
# is odd, the last number is 2 or 6.
define Div4  [4 | 8 | N* [Even [%0 | 4 | 8] | Odd [2 | 6]]];

# Leap years are divisible by 4 but we subtract centuries
# that are not divisible by 400. Note the double
# subtraction. [[N+ - Div4] {00}] includes 1900 but not 2000.
define LeapYear [Div4 - [[N+ - Div4] {00}]] ;

# Note the [.#. | \N] at the end of the right context.
# 2005 is not a leap year although the year 200 was (in principle).
define LeapDates {February 29, } => _ LeapYear [.#. | \N];

define ValidDates [AllDates & MaxDays & LeapDates];

define DateParser [ValidDates @-> "<DATE>" ... "</DATE>"] ;

echo
echo Testing DateParser

push DateParser

down Today is Wednesday, October 5, 2004.
down Yesterday was Tuesday.
down February 29, 2000 was a leap day.
down but February 29, 1900 was not a leap day.
down Next leap day is on February 29, 2008.