the following line:

$line = str_replace("pattern", Pattern(),$line);

where Pattern() is a function that returns a string.

Pattern executes everytime this code line is encountered, even if "pattern" is not a match on $line.

so if i loop through a file running that line, the function Pattern() will execute every single time, match or no match.

(my) common sense says this function should only execute when a match is found.  Is this a bug? I didn't know about it and something like this could seriously affect performance of a program.

Also I don't think the modules or any other of your requirements are relevant for this issue.