B after A

You are given a string, $str. Write a script to return true if there is at least one b, and no a appears after the first b.

Examples

Example 1
Input: $str = "aabb"
Output: true

Example 2
Input: $str = "abab"
Output: false

Example 3
Input: $str = "aaa"
Output: false

Example 4
Input: $str = "bbb"
Output: true

Analysis

This is an easy one-liner:

($str =~ m|b(.*)| and $1 =~ m|^[^a]*$|) ? 
   'true' : 'false'

The first regular expression - $str =~ m|b(.*)| - looks for a 'b' and the second - $1 =~ m|^[^a]*$| - checks for no 'a' in the part of the string follwing the first 'b': note that the $1 refers to the matched substring from the first regular expression.

Perl pure regex implementation to deal with both tasks. Don't forget to try DIY tool.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2024-06-10
use utf8;     # Week 273 - task 2 - B after a
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';

b_after_a('aabb');
b_after_a('abab');
b_after_a('aaa');
b_after_a('bbb');
b_after_a('ccc');
b_after_a('Write a script to return true if there is at least one b');

sub b_after_a {
    
    my $str = $_[0];
    
    # match a 'b', and then match no 'a' in the rest of the string
    say qq[\nInput:  \$str = '$str'];
    say qq[Output: ] . (($str =~ m|b(.*)| and $1 =~ m|^[^a]*$|) ? 'true' : 'false');
}

4 lines of code

Output from script

Input:  $str = 'aabb'
Output: true

Input:  $str = 'abab'
Output: false

Input:  $str = 'aaa'
Output: false

Input:  $str = 'bbb'
Output: true

Input:  $str = 'ccc'
Output: false

Input:  $str = 'Write a script to return true if there 
   is at least one b'
Output: true