Programming Perl, 4th Edition

Where You Left Off: The \G Assertion

Whenever you start thinking in terms of the pos function, it’s tempting to start carving your string up with substr, but this is rarely the right thing to do. More often, if you started with pattern matching, you should continue with pattern matching. However, if you’re looking for a positional assertion, you’re probably looking for \G.

The \G assertion represents within the pattern the same point that pos represents outside of it. When you’re progressively matching a string with the /g modifier (or you’ve used the pos function to directly select the starting point), you can use \G to specify the position just after the previous match. That is, it matches the location immediately before whatever character would be identified by pos. This allows you to remember where you left off:

($recipe = <<'DISH') =~ s/^\s+//gm;
    Preheat oven to 451 deg. Fahrenheit.
    Mix 1 ml. dilithium with 3 oz. NaCl and
    stir in 4 anchovies.  Glaze with 1 g.
    mercury.  Heat for 4 hours and let cool
    for 3 seconds.  Serves 10 aliens.
DISH

$recipe =~ /\d+ /g;
$recipe =~ /\G(\w+)/;           # $1 is now "deg"
$recipe =~ /\d+ /g;
$recipe =~ /\G(\w+)/;           # $1 is now "ml"
$recipe =~ /\d+ /g;
$recipe =~ /\G(\w+)/;           # $1 is now "oz"

The \G metasymbol is often used in a loop, as we demonstrate in our next example. We “pause” after every digit sequence, and, at that position, we test whether there’s an abbreviation. If so, we grab the next two words. Otherwise, we just grab the next word:

pos($recipe) = ...