Programming Perl, 4th Edition

study

study SCALAR
study

This function takes extra time to study SCALAR 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 runtimes with and without it to see which runs faster. Those loops that scan for many short constant strings (including the constant parts of more complex patterns) will benefit most from study. If all your pattern matches are constant strings anchored at the front, study won’t help at all because no scanning is done. 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 /\bglarch\b/; ... print; ...