Perl5 リファレンス

       study SCALAR

       study   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 (<>) {
                       print ".IX foo\n"       if /\bfoo\b/;
                       print ".IX bar\n"       if /\bbar\b/;
                       print ".IX blurfl\n"    if /\bblurfl\b/;
                       # ...

               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

               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";