One more way.
#!/usr/bin/perl
use warnings;
use strict;
my %huge_trans = (
zero => 0,
one => 1,
ones => 11,
two => 2,
three => 3,
# ...
million => 1_000_000,
);
my $rx_word = qr/\b\w+\b/i;
print <> =~ s!($rx_word)! $huge_trans{lc $1} // $1 !ger;
Imagine you have huge dictionary for translation. Then this way should work faster.
Previously suggested way with alternations should also work fast, because Trie-optimization should kick-in.
Upd. BTW, it is not about "combining regex'es", it's only a use of a hash. All words, i.e. hash keys, must be simple words, not regexes.