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Programming Perl

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3.2.72 grep

grep EXPR, LIST
grep BLOCK LIST

This function evaluates EXPR or BLOCK in a Boolean context for each element of LIST, temporarily setting $_ to each element in turn. 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 doesn't have to be.) In scalar context, grep returns the number of times the expression was true.

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

@code_lines = grep !/^#/, @all_lines;

Since $_ is a reference into the list value, altering $_ will modify 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"! Caveat Programmor.

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

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


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