Learning Perl, Fourth Edition

Option Modifiers

Several option modifier letters, sometimes called flags , may be appended as a group right after the ending delimiter of a regular expression to change its behavior from the default.

Case-Insensitive Matching with /i

To make a case-insensitive pattern match, so you can match FRED as easily as fred or Fred, use the /i modifier:

    print "Would you like to play a game? ";
    chomp($_ = <STDIN>);
    if (/yes/i) {  # case-insensitive match
      print "In that case, I recommend that you go bowling.\n";
    }

Matching Any Character with /s

By default, the dot (.) doesn’t match newline, and this makes sense for most “look within a single line” patterns. If you have newlines in your strings and want the dot to be able to match them, the /s modifier will do the job. It changes every dot^[191] in the pattern to act as the character class [\d\D] does, which is to match any character, even if it is a newline. You have to have a string with newlines for this to make a difference:

    $_ = "I saw Barney\ndown at the bowling alley\nwith Fred\nlast night.\n";
    if (/Barney.*Fred/s) {
      print "That string mentions Fred after Barney!\n";
    }

Without the /s modifier, that match would fail since the two names aren’t on the same line.

Adding Whitespace with /x

The /x modifier allows you to add arbitrary whitespace to a pattern to make it easier to read.

    /-?\d+\.?\d*/         # what is this doing?
    / -? \d+ \.? \d* /x   # a little better

Since /x allows whitespace inside the pattern, a literal space or tab character within the pattern ...