Learning Perl, 5th Edition

Option Modifiers

There are several option modifier letters, sometimes called flags, which 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 might have newlines in your strings, and you want the dot to be able to match them, the /s modifier will do the job. It changes every dot^[*] in the pattern to act like the character class [\d\D] does, which is to match any character, even if it is a newline. Of course, 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 third modifier you’ll see 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 the /x allows whitespace inside the pattern, ...