Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified. This may or may not
save time, depending on the nature and number of patterns you are searching on, and on the
distribution of character frequencies in the string to be searched--you probably want to
compare run times with and without it to see which runs faster. Those loops which scan for
many short constant strings (including the constant parts of more complex patterns) will
benefit most. You may have only one study active at a time--if you study a
different scalar the first is "unstudied". (The way study works is
this: a linked list of every character in the string to be searched is made, so we know,
for example, where all the 'k' characters are. From each search string, the
rarest character is selected, based on some static frequency tables constructed from some
C programs and English text. Only those places that contain this "rarest"
character are examined.)
For example, here is a loop that inserts index producing entries before any line
containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
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In searching for /\bfoo\b/, only those locations in $_ that
contain f will be looked at, because f is rarer than o.
In general, this is a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the first place.
Note that if you have to look for strings that you don't know till runtime, you can
build an entire loop as a string and eval that to avoid recompiling all your
patterns all the time. Together with undefining $/ to input entire files as
one record, this can be very fast, often faster than specialized programs like fgrep(1).
The following scans a list of files (@files) for a list of words (@words),
and prints out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
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