#!/usr/bin/perl
use strict;
use warnings;
my $string = 'foo 1foo;   foo_2   foo-bar() 87   - _ !@#$% ';
my $optional  = qr/[\d_-]/;
my $mandatory = qr/[a-zA-Z]/;
$string =~  s/$optional*$mandatory$optional*//g;
print "$string\n";