Grep could have been called "filter". But it is: "get regular expression".
Use grep when you think about getting a subset of an array.
use strict;
use warnings;

my @x = ("aaBxyz", "..Bxys", "bbxyzzy", "xxAx");

# for each element in the array @x, 
# pass that element to the array @result if
# if the 3rd character is either A or B
# followed by at least one character.

my @result = grep {/^..(A|B)./}@x;

print "$_\n" for @result;

=prints
aaBxyz
..Bxys
xxAx
=cut

[download]
Yes, in this case $1 will be either A or B if the regex is True.
I can show that, but I don't want to confuse you.
Here $1 is not relevant.

Ok, on second thought, here we go... 

#ok, with a weird example of $1
@result = grep{ print "$1 $_\n" if /^..(A|B)./;
                /^..(A|B)./}@x;
                
=prints
B aaBxyz
B ..Bxys
A xxAx
=cut
@result is the same as above

# grep {} executes the code within the brackets and passes the input
# to @result depending upon the true/false value of the last statement
+.

[download]

In reply to Re: do $n exist in grep? by Marshall
in thread do $n exist in grep? by misterperl

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