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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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grep BLOCK LIST

grep EXPR,LIST

This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it is not limited to using regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value consisting of those elements for which the expression evaluated to true. In scalar context, returns the number of times the expression was true.

@foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

@foo = grep {!/^#/} @bar;    # weed out comments

Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Similarly, grep returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by grep (for example, in a foreach, map or another grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.

If $_ is lexical in the scope where the grep appears (because it has been declared with my $_) then, in addition to being locally aliased to the list elements, $_ keeps being lexical inside the block; i.e., it can't be seen from the outside, avoiding any potential side-effects.

See also for a list composed of the results of the BLOCK or EXPR.

ISBN 9781906966027Perl Language Reference ManualSee the print edition