Programming Perl, 4th Edition

Progressive Matching

When used with the /g modifier, the pos function allows you to read or set the offset where the next progressive match will start:

$burglar = "Bilbo Baggins";
while ($burglar =~ /b/gi) {
    printf "Found a B at %d\n", pos($burglar)–1;
}

(We subtract one from the position because that was the length of the string we were looking for, and pos is always the position just past the match.)

The code above prints:

Found a B at 0
Found a B at 3
Found a B at 6

After a failure, the match position normally resets back to the start. If you also apply the /c (for “continue”) modifier, then when the /g runs out, the failed match doesn’t reset the position pointer. This lets you continue your search past that point without starting over at the very beginning.

$burglar = "Bilbo Baggins";
while ($burglar =~ /b/gci) {        # ADD /c
    printf "Found a B at %d\n", pos($burglar)–1;
}
while ($burglar =~ /i/gi) {
    printf "Found an I at %d\n", pos($burglar)–1;
}

Besides the three Bs it found earlier, Perl now reports finding an i at position 10. Without the /c, the second loop’s match would have restarted from the beginning and found another i at position 1 first.