### https://theweeklychallenge.org/blog/perl-weekly-challenge-347/
"""

Task 2: Format Phone Number

Submitted by: [45]Mohammad Sajid Anwar
     __________________________________________________________________

   You are given a phone number as a string containing digits, space and
   dash only.

   Write a script to format the given phone number using the below rules:
1. Removing all spaces and dashes
2. Grouping digits into blocks of length 3 from left to right
3. Handling the final digits (4 or fewer) specially:
   - 2 digits: one block of length 2
   - 3 digits: one block of length 3
   - 4 digits: two blocks of length 2
4. Joining all blocks with dashes

Example 1

Input: $phone = "1-23-45-6"
Output: "123-456"

Example 2

Input: $phone = "1234"
Output: "12-34"

Example 3

Input: $phone = "12 345-6789"
Output: "123-456-789"

Example 4

Input: $phone = "123 4567"
Output: "123-45-67"

Example 5

Input: $phone = "123 456-78"
Output: "123-456-78"
     __________________________________________________________________

   Last date to submit the solution 23:59 (UK Time) Sunday 16th November
   2025.
     __________________________________________________________________

SO WHAT DO YOU THINK ?
"""
### solution by pokgopun@gmail.com

def fpn(string: str) -> str:
    dgt = [e for e in string if e.isdigit()]
    #print(dgt)
    l = len(dgt)
    i = 3
    while i < l:
        if l-i == 1:
            i -= 1
        dgt[i] = "-" + dgt[i]
        i += 3
    #print(dgt)
    return "".join(dgt)

import unittest

class TestFpn(unittest.TestCase):
    def test(self):
        for inpt, otpt in {
                "1-23-45-6": "123-456",
                "1234": "12-34",
                "12 345-6789": "123-456-789",
                "123 4567": "123-45-67",
                "123 456-78": "123-456-78",
                }.items():
            self.assertEqual(fpn(inpt),otpt)

unittest.main()