I’m trying to match a sentence fragment of the form “after 3 days” or “after 1 month”. I want to be particular with the single and plural forms, so “1 day” is valid but “1 days” is not.

I have the following code which is nearly there but the first two entries in the failure tests don’t fail. Any suggestions please that use syntactical notation as I’d like, if possible, to avoid a set_parse_action() that checks the numeric value against the unit’s plurality.

from pyparsing import *

units = Keyword('days') ^ Keyword('months')
unit  = Keyword('day') ^ Keyword('month')

single = Literal('1') + unit
multi = Word(nums) + units

after = Keyword('after') + ( single ^ multi )

a = after.run_tests('''
    after 1 day
    after 2 days
    after 1 month
    after 2 months
    ''')

print('=============')

b = after.run_tests('''
    after 1 days
    after 2 day
    after 1day
    after 2days
    ''', failure_tests = True)

print('Success tests', 'passed' if a[0] else 'failed')
print('Failure tests', 'passed' if b[0] else 'failed')