Let me point out a couple notes on using regexes to solve this problem; first, though, using modulus is definitely the way to go (it averages between 3 and 2.5 times faster than regex approaches). It is important, when given a language such as Perl, to know which tools are hammers, screwdrivers, and wrenches; and even moreso, which problems are nails, screws, and bolts. This is a case where you are using a sledgehammer (a regex) to flatten a piece of wood, when a simple piece of sandpaper (the % operator) will do.

There are four generic approaches to this:

/(\d)*(\d)/
useless capturing of preceeding digits; actually captures one digit at a time over and over again; useless backtracking is forced; 2.96 x slower than modulus

/(\d*)(\d)/
useless capturing of preceeding digits; useless backtracking is forced; 2.80 x slower than modulus

/\d*(\d)/
useless backtracking is forced; 2.79 x slower than modulus

/(\d)$/
optimized (goes to end of string automatically); 2.54 x slower than modulus
Code used for benchmarks: 
use Benchmark 'timethese';

$x = int (1_000_000 * rand 1_000_000);

timethese(-5, {
  multiple    => sub { $x =~ /(\d)*(\d)/ },
  backtrack_c => sub { $x =~ /(\d*)(\d)/ },
  backtrack   => sub { $x =~ /\d*(\d)/   },
  opt         => sub { $x =~ /(\d)$/     },
  mod         => sub { $x % 10           },
});

[download]
If snafu doesn't mind, I'd like to use this as an example in my book -- first to show how to craft a regex, then to show why there are some places where a regex is overkill.

japhy -- Perl and Regex Hacker

In reply to japhy regex analysis: case study (RE question...) by japhy
in thread RE question...yup, another one ;) by snafu

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