/x
modifier to add comments and whitespace to our regular
expressions. We can also do this with the (?#)
pattern:
/^Today's (?# This is ignored, by the way)date:/
Unfortunately, there's no way to have parentheses inside these
comments, since perl closes the comment as soon as it sees a
closing bracket. If you want to have longer or more detailed
comments, you should consider using the /x modifier
instead.
Inline Modifiers
If you are reading patterns from a file or constructing them from
inside your code, you have no way of adding a modifier to the end
of the regular expression operator. For example:
#!/usr/bin/perl
# inline.plx
use warnings;
use strict;
my $string = "There's more than One Way to do it!";
print "Enter a test expression: ";
my $pat = <STDIN>;
chomp($pat);
if ($string =~ /$pat/) {
print "Congratulations! '$pat' matches the sample string.\n";
} else {
print "Sorry. No match found for '$pat'";
}
If we run this and momentarily forgot how our sample string had
been capitalized, we might get this:
>perl inline.plx
Enter a test expression: one way to do it!
Sorry. No match found for 'one way to do it!'
>
So how can we make this case-insensitive? The solution is to use
an inline modifier, the syntax for which is (?i) .
This will make the enclosing group match case-insensitively.
Therefore we have:
>perl inline.plx
Enter a test expression: (?i)one way to do it!
Congratulations! '(?i)one way to do it!' matches the sample string.
>
If, conversely, you have a modifier in place that you temporarily
want to get rid of, you can say, for example, (?-i)
to turn it off. If we have this:
/There's More Than ((?-i)One Way) To Do It!/i;
the words 'One Way' alone are matched case-sensitively.
Note that you can also inline the /m , /s,
and /x modifiers in the same way.
Grouping without Backreferences
Parentheses perform the function of grouping and populating the
backreference variables. If you have a portion of your match in
parentheses, it will, if successful, be placed in one of the
numbered variables. However, there may be times when you only
want to use brackets for grouping. For example, you're expecting
the first backreference to contain something important, but there
may be some preceding text in the way. You could have something
like this:
/(X-)?Topic: (\w+)/;
You can't be certain whether your first defined backreference is
going to end up in $1 or $2 - it
depends on whether the 'X-' part is present or not.
For example, if we tried to match the string "Topic: the
weather", we'd find that $1 was left
undefined. If we'd tried to do something with its contents, we'd
get the warning:
Use of uninitialized value in concatenation
Now that's not necessarily a problem here. After all, we'll find
our word in $2 whether or not there's anything
preceding "Topic:". Surely we can just be
careful not to use $1?
But what if there's more than one optional field? Say we had an
expression that left all but the 2nd and 6th groups optional. We
then have to look in $2 for our first word and $6 for our second,
while $1, $3, $4, and $5 are left undefined. This really isn't
good programming style and is asking for trouble! We really
shouldn't backreference fields if we don't need to. We can
resolve this problem very simply, by adding the characters
?: like this:
/(?:X-)?Topic: (\w+)/;
This ensures that the first set of brackets will now group only
and not fill a backreference variable. Our word will always be
put into $1.
Transliteration - Page 16
Beginning Perl
Lookaheads and Lookbehinds - Page 18
