my @values = qw/ a ab. d ef def g|h /;
my ($regex) = map { qr/$_/ }          # 5.
    join '|',                         # 4.
    map {quotemeta}                   # 3.
    sort { length $b <=> length $a }  # 2.
    @values;                          # 1.
print "$regex\n";                     # 6.

##</code><code>##

(?^:ab\.|def|g\|h|ef|a|d)

##</code><code>##

if ($input=~$regex) {
    print "It matches!\n";
}
# or
while ($input=~/($regex)/g) {
    print "Found string: $1\n";
}

##</code><code>##

my %map = ( a=>1, ab=>23, cd=>45 );   # 1.
my ($regex) = map { qr/$_/ }          # 2.
    join '|', map {quotemeta}
    sort { length $b <=> length $a
           or $a cmp $b }             # 3.
    keys %map;
print "$regex\n";                     # 4.
# Now, use the regex
my @strings = qw/ abcd aacd abaab /;  # 5.
for (@strings) {
    my $before = $_;
    s/($regex)/$map{$1}/g;            # 6.
    print "$before -> $_\n";          # 7.
}

##</code><code>##

abcd -> 2345
aacd -> 1145
abaab -> 23123