Format phone number

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

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

Examples

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'

Analysis

I wonder where this is the standard way of formatting phone numbers? For that matter, I wonder how many people now use phone numbers other than just as a string of digits?

But that's not what we're asked.

In my experience theses sorts of problems are most efficiently solved using substr and length, so that's what I did.

You could undoubtedly do it using a regular expression, but it's always messy using one when there's a decision that depends on the end of the string rather than the start.

This is a practical, no-nonsense Perl implementation that focuses on getting the job done with clean, readable code. Peter demonstrates solid Perl skills with a straightforward approach.

This review may cover either or both challenges for this week.

I am sorry that the 'Try it' feature is currently working very slowly or not at all owing to some issue with my web hosting provider.

Try it 

Try running the script with any input:

Your $phone =

example: 3 14159 26535

⤡

#!/usr/bin/perl

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

use v5.26;    # The Weekly Challenge - 2025-11-10
use utf8;     # Week 347 - task 2 - Format phone number
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

format_phone_number('1-23-45-6');
format_phone_number('1234');
format_phone_number('12 345-6789');
format_phone_number('123 4567');
format_phone_number('123 456-78');
format_phone_number('02072221212');

sub format_phone_number {
    
    my ($phone, $output, $length);
    
    # initialise
    $phone = shift;
    say qq[\nInput:  '$phone'];
    $phone =~ s|[^[0-9]||g;
    
    # groups of 3
    while (length($phone) > 4) {
        $output .= substr($phone, 0, 3) . '-';
        $phone = substr($phone, 3);
    }
    
    # and the rest
    $output .= length($phone) == 4 
        ? substr($phone, 0, 2) . '-' . substr($phone, 2, 2) 
        : $phone;
    
    say qq[Output: '$output'];
}

12 lines of code

Output from script

Input:  '1-23-45-6'
Output: '123-456'

Input:  '1234'
Output: '12-34'

Input:  '12 345-6789'
Output: '123-456-789'

Input:  '123 4567'
Output: '123-45-67'

Input:  '123 456-78'
Output: '123-456-78'

Input:  '02072221212'
Output: '020-722-212-12'