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

{
    my @classes = (

        ['[a-zA-Z]' => '\w'],
        ['[0-9]' => '\d'],
        ['\w' => '_'], #that's why order matters
        ['.' => '.'],
    );

    sub make_regex {
        local $_ = @_ ? shift : $_;
        my $result = '';

        my $i = -1;
        while( ++$i < @classes ){

            my $p = pos($_) || 0;

            my ($re,$su) = @{ $classes[$i] };

            if( /\G($re+)/g ){

                $result .= $su . '{' . length($1) . '}';
                $i = -1;
            }
            else {
                pos($_) = $p;
            }

        }

        $result
    }
}

printf "%s => %s\n",$_,make_regex for (
    'abc12','123','#+#+#',
)