Programming Perl, 4th Edition

grep

grep EXPR, LIST
grep BLOCK LIST

This function evaluates EXPR or BLOCK in Boolean context for each element of LIST, temporarily setting $_ to each element in turn, much like the foreach construct. In list context, it returns a list of those elements for which the expression is true. (The operator is named after a beloved Unix program that extracts lines out of a file that match a particular pattern. In Perl, the expression is often a pattern, but it doesn’t have to be.) In scalar context, grep returns the number of times the expression was true.

If @all_lines contains lines of code, this example weeds out comment lines:

@code_lines = grep !/^\s*#/, @all_lines;

Because $_ is an implicit alias to each list value, altering $_ modifies the elements of the original list. While this is useful and supported, it can occasionally cause bizarre results if you aren’t expecting it. For example:

@list = qw(barney fred dino wilma);
@greplist = grep { s/^[bfd]// } @list;

@greplist is now “arney”, “red”, “ino”, but @list is now “arney”, “red”, “ino”, “wilma”! Ergo, Caveat Programmor.

See also map. The following two statements are functionally equivalent:

@out = grep { EXPR } @in;
@out = map { EXPR ? $_ : () } @in

For a version of grep that short circuits, see the first function from the standard List::Util module. Instead of returning a list of all elements for which the EXPR was true, it returns only the first such, or undef if none were. As always, $_ is set to each element:

use List::Util qw(first); ...