# /a\d+b/
while (<DATA>) {
   local our $incomplete;
   my $match = /
      a
      (?:$(?{$incomplete=1})(?!)|(?(?{$incomplete})(?!))
         \d+
         (?:$(?{$incomplete=1})(?!)|(?(?{$incomplete})(?!))
            b
         )
      )
   /x;

   my $rv = $match      ? "match"
          : $incomplete ? "incomplete"
          :               "no match";

   chomp;
   printf("%-10s  %s\n", $_, $rv);
}

__DATA__
a123b
a
a1
a123
a123c
a123ca123b
a123ca123
a123ca123c

##</code><code>##

a123b       match
a           incomplete
a1          incomplete
a123        incomplete
a123c       no match
a123ca123b  match
a123ca123   incomplete
a123ca123c  no match

##</code><code>##

my $ws = qr/\s+/;

sub get_token {
   my ($self) = @_;
   for ($self->{buf}) {
      s/^$ws//;
      if (length() == 0) {
         my $fh = $self->{fh};
         return [ TOK_EOF ] if eof($fh);
         $_ .= <$fh>;
         redo;
      }

      s/^([a-zA-Z][a-zA-Z0-9_]*)// && return [ TOK_IDENT, $1 ];
      ...
   }
}