#!/usr/local/bin/perl
#
use strict;
use warnings;

our @tryThese = (
   "a 10\nb a2 s2\nc 30",
   "a 10\nb\nc 30",
   "a 10\nb  \nc 30",
   "a 10\nb a2 s2  \nc 30");

our $c;

our $rxAfterB = qr{(?m)^b[\x20\x09]*([^\n]*)};

##</code><code>##

foreach my $a (@tryThese)
{
    print "\n\$a contains ...\n";
    {
        local $" = "<--\n";
        print "@{[split /\n/, $a]}\n";
    }
    ($c) = $a =~ /$rxAfterB/;
    print "c is -->$c<--\n\n";
}

##</code><code>##

$a contains ...
a 10<--
b a2 s2<--
c 30
c is -->a2 s2<--


$a contains ...
a 10<--
b<--
c 30
c is --><--


$a contains ...
a 10<--
b  <--
c 30
c is --><--


$a contains ...
a 10<--
b a2 s2  <--
c 30
c is -->a2 s2  <--

##</code><code>##

    ($c) = $a =~ $rxAfterB;