| Lesson 11 | Perl special $_ variable |
| Objective | Write a program that uses the pattern-matching operators with a yes/no question. |
Perl Pattern Matching Variable
There is one item we need to cover about the pattern-matching operators before you actually start to use them.
The three operators discussed in the last several lessons use the special $_ variable by default.
The $_ variable is the default input and pattern-matching workspace. It is implicitly set in a while loop when the only parameter is a file input. In other words, these two loops are equivalent:
while(<>) { ... }
while($_ = <>) { ... }
Many functions and operators work with $_ by default to make certain common tasks easier to write. In fact, all of our examples in this lesson so far have used $_ as the pattern-matching space.
In order to use a different variable with these pattern-matching operators, you need use the =~ operator to specify the variable to use.
For example, you could use the match operator like this:
while($line = <>) {
print $line if $line =~ /hello/i;
}
Or, you could use the substitute operator like this:
while($line = <>) {
$line =~ s/hello/Goodbye/ig;
print $line;
}
Or, the translation operator could be done like this:
$line =~ tr/A-Z/a-z/;
When used in a logical sense (as in the match operator above), you can use !~ to negate the logical value.
This example will print all the lines that do not have "hello" in them:
while($line = <>) {
print $line if $line !~ /hello/i;
}
Pattern Matching Operator - Exercise